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Abstract

Recent studies have focused on developing new materials capable of withstanding diverse environmental or weathering conditions, including sunlight, temperature, moisture, and chemicals, for various outdoor applications. Nowadays, nanotechnological advancements have opened possibilities for the development of polymer nanocomposites, which exhibit superior mechanical and weathering properties over traditional materials. However, there is limited literature available that summarizes the improvements in the mechanical and weathering properties of nanocomposites specifically for FFF. This paper provides a concise review of composite material development through the incorporation of nanofillers (NFs) to enhance product strength and durability. The primary focus is on examining the effect of the inorganic nanofillers, especially metal oxides NFs (TiO₂, ZnO, SiO₂), on the mechanical and weathering properties of the polymers in different manufacturing processes. Paper also covered the challenges and opportunities along with the applications of metal oxide-based polymer nanocomposites for outdoor use.

Keywords

Polymer nanocomposites metal oxide nanofillers nano-silica zinc oxide titanium dioxide environment conditions additive manufacturing fused filament fabrication

Introduction

With the continuous increase in demand for customized products or components using advanced manufacturing technologies in the modern world, there is a need to study the impact of the environmental conditions on their performance for better sustainability. The demand for sustainable products or components has necessitated a focus on ensuring reliable performance, high efficiency, and prolonged durability under diverse environmental conditions such as varying temperatures, sunlight exposure, moisture, and exposure to chemicals. The mechanical and surface properties of the materials used in manufacturing directly impact a component’s characteristics in varying environmental conditions. Additionally, the design and manufacturing processes play pivotal roles that influence the reliability of a product or component’s performance in different applications. Thus, the researchers increase interest in developing an effective material that will sustain under different environmental exposures.

Additive manufacturing (AM) and polymer materials

Additive Manufacturing (AM), also known as 3D Printing (3DP), has emerged as an advanced manufacturing technologies capable of fabricating components with complex geometries through the layer-by-layer addition of materials. Nowadays, the implementation of AM processes is rapidly growing due to their flexibility and design freedom. The component manufacturing capabilities of AM, with any complexity, offer significant savings in cost and materials with the shortest production time. Components manufactured using AM are being used in various sectors, such as the automotive, biomedical, electronics, aerospace, robotics, and manufacturing industries.^1,2 One major constraint of AM is the limited range of available processable materials and their inadequate properties. Although in the last decade revolutionary changes have occurred in AM, research and development of composite materials is one of the major changes in AM for various manufacturing industries. Consequently, researchers have been focusing on developing novel AM materials such as composites, nanocomposites, functionally graded materials, and multifunctional materials with desirable properties.^3,4

A variety of materials have been used in AM, including metals, polymers, ceramics, and their composites. Polymers and their composites are widely used in AM owing to their inherent characteristics, such as lightweight nature, easy processability, viable mechanical, thermal, and electrical properties, as well as wear resistance, flame retardancy, and corrosion resistance (chemicals, moisture).⁵ Polymer materials, especially thermoplastic, are some of the most widely used materials for different application fields over other traditional materials. Polymeric products are exposed to different environmental conditions, such as sunlight, moisture, temperature, and chemicals, which can lead to changes in their physical, chemical, and mechanical properties. These environmental exposures can significantly impact microstructure, physical appearance (color change, surface erosion), and mechanical properties (such as strength and modulus), ultimately affecting product durability.^6,7 Recently, the growing interest in studying the effect of the environmental or weathering conditions on AM polymeric products has opened the way for researchers to consider product durability alongside with mechanical performance. This shift in focus, aims to extend the service life of polymeric products manufactured using AM. Therefore, it is vital to develop materials that are sustainable under outdoor environmental exposure processed through the AM processes.

According to the ASTM F42 standard, AM are classified into seven different categories based on their working principles, that is, material extrusion (ME), vat photo-polymerization (VP), direct energy deposition (DED), binder jetting (BJ), powder bed fusion (PBF), material jetting (MJ), and sheet lamination (SL). These AM techniques have their own characteristics, such as material availability, speed, print resolution, and cost, which makes them suitable for use in different industries accordingly. Various AM techniques are available to manufacture polymeric products for different industries. Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS), and Stereolithography (SLA) are commonly used techniques for processing polymers in AM (Table1).

Table 1.

Techniques for polymer processing in AM.

Technique	Working principles	Materials used
Fused Filament Fabrication (FFF)	- Semi-solid thermoplastic filament is heated and extruded through a nozzle, depositing material layer by layer to build the solid part	Thermoplastics filaments – ABS, PA, PLA, PP, PET-G, PC, PEEK and ULTEM
Selective Laser Sintering (SLS)	- Thermal energy (focused laser radiations) selectively fuses the material depending upon the design of the fabricated object	Powder materials – Polymers (PA (nylon), PS, TPE, PAEK), Metals (aluminum, titanium, steel), Ceramics
Stereolithography (SLA)	- Photo-sensitive materials are exposed to radiation/light in a controlled manner to obtain polymerized material layers. Subsequent layers combine to form an object	Photopolymer resins (monomers, oligomers/binders, photoinitiators, and additives - nanoparticles or pigmentation)

Among these techniques, FFF is the most extensively used; it is also referred to as Fused Deposition Modeling (FDM). In the FFF process, components are built by extruding material in a semi-solid state through a nozzle, depositing it in a successive manner on a platform until the final 3D parts are produced (Figure 1).⁹ Thermoplastic polymers are more often used as input materials in the FFF process. Acrylonitrile Butadiene Styrene (ABS), Polyamide (PA), Polylactic Acid (PLA), Polypropylene (PP), Polyethylene Terephthalate Glycol (PET-G), Polyether Ether Ketone (PEEK), and Polycarbonate (PC) are some of the widely used thermoplastic polymers in the FFF process.¹⁰ While FFF enables the processing of a wide range of materials, commercially available FFF materials often struggle to perform optimally under different environmental or weathering conditions as shown in Figure 1. There is limited research on the effects of different environmental or weathering conditions, such as sunlight (UV radiation), moisture, elevated temperature, and chemicals, on these polymers in various outdoor application fields (aerospace, biomedical, automotive, and marine).^11,12 Table 2 provides a concise overview for readers on how FFF-fabricated parts have performed under different environmental or weathering conditions till date, along with the effects on the properties of FFF parts. The literature studies conclude that different materials such as ABS, PA (Nylon), PLA, HIPS, TPU, except PEEK, PETG significantly reduce the mechanical and surface properties of FFF-manufactured parts under different environmental conditions. The reduction in material properties of the polymer is particularly impactful due to the strong interconnections between different types of degradation (photo-oxidation, thermal, and hydrolysis). The combined effects of environmental exposures significantly impact the microstructure, appearance, and mechanical properties of FFF- fabricated polymeric parts, ultimately influencing their durability.

Figure 1.

Schematic of FFF process and environmental or weathering conditions.⁸

Table 2.

Summary of studies related to the effect of environmental or weathering conditions on FFF parts.

Author (Year)	Polymer matrix + nanofillers	Environmental condition	Effect of environmental conditions on properties	Ref
Magri et al. (2023)	PEEK/PEI blend	Moisture, heat, and chemical solvents	- Slightly reduce elastic properties	¹³
			- No substantial change in degree of crystallinity, tensile strength, thermal properties
			- Blend remains stable under aging conditions
Banjo et al. (2022)	Nylon 6, CF-Nylon 6, PLA	Moisture and Elevated temperature	- Degree of crystallinity reduced	¹⁴
Banjo et al. (2022)	Nylon 6, CF-Nylon 6, PLA	Moisture and Elevated temperature	- Significant decrease in mechanical properties	¹⁴
Thomas et al. (2021)	PETG	UV radiation, temperature, and humidity	- Creep behaviour increases under UV exposure 100 h whereas in other conditions creep behaviour decreases	¹⁵
Thomas et al. (2021)	PETG	UV radiation, temperature, and humidity	- UV exposure up to 200 h results higher creep resistance	¹⁵
Eva et al. (2021)	TPE, TPU	Abrasive fluid (commercial automotive petrol)	- Effect of chemical abrasive (petrol 98) greatly reduces tension, modulus of elasticity and hardness of parts manufactured using TPE, TPU	¹⁶
Ana et al. (2021)	ABS, HIPS, TPU, PMMA	Artificial saliva	- Aging condition didn’t affect significantly on mechanical properties of mono-materials, except for PMMA, flexural strength decreased	¹⁷
Amza et al. (2021)	PLA, PETG	Ultraviolet UV-B radiation	- Significant reductions in tensile strength (PLA: −5.3%; PETG: −36%) and compression strength (PLA: −6.3%; PETG: −38.3%)	¹⁸
Nieto et al. (2021)	PLA, PETG	Water Environments	- PETG specimens stable in all solutions, slight weight variation around 0.3% after 9 weeks	¹⁹
Nieto et al. (2021)	PLA, PETG	Water Environments	- PLA specimens weight increases around 2.5% after 8 weeks	¹⁹
Amirpasha et al. (2021)	PLA	Laboratory conditions (RT, and humidity); aqueous environment	- In both orientation, tensile strength degraded under environmental conditions	²⁰
Bruno et al. (2020)	CF-Nylon	Moisture	- Moisture significantly affects the interface of the fiber and matrix, and the adhesion between printed roads or layers	²¹
Bruno et al. (2020)	CF-Nylon	Moisture	- Significantly decrease in mechanical properties	²¹
Leena et al. (2020)	PEEK	Ambient and cryogenic environments	- PEEK showed lower the strength and modulus, but increase in elongation (150–250%)	²²
Leena et al. (2020)	PEEK	Ambient and cryogenic environments	- At ambient environments, PEEK shows ductile fracture behavior at 30 $℃$ and brittle fracture behavior at −197 $℃$	²²
William et al. (2020)	Fiber reinforced ABS	Heat or moisture	- Specimens under temperature conditioned below T_g, the polymer chain shows a slight reduction in the UTS and a significant change in the elongation to break	²³
William et al. (2020)	Fiber reinforced ABS	Heat or moisture	- Specimens subjected to moisture exposure increased variability and reduced tensile strength and elongation to break	²³
Afshar et al. (2020)	ABS-Cu	UV radiation and moisture	- ABS specimens significantly reduce mechanical properties under the exposure of UV radiation and moisture	²⁴
Afshar et al. (2020)	ABS-Cu	UV radiation and moisture	- Copper coating-ABS specimens provides good protection, retaining flexural strength, ductility, and toughness under the exposure of UV radiation and moisture	²⁴

For outdoor applications, polymer materials must be resistant to these environmental or weathering conditions while maintaining high mechanical strength at a cost-effective price.²⁵ Currently available commercial polymers for FFF may not perform adequately in outdoor applications, but this can be improved by introducing various types of fillers to achieve the desired properties.^26,27 The addition of fillers in the polymer matrix imparts desirable properties, as depicted in Figure 2, which increases their use in a various application. Therefore, there is a need to develop composite materials that possess enhanced mechanical and weathering properties (i.e., UV, moisture, chemical and heat resistance) for different environmental or weathering conditions. The incorporation of nanomaterials into the polymer matrix to develop composite materials will be the potential solution to address the effects of different environmental or weathering conditions.

Figure 2.

Desirable properties of composite materials.

Polymer nanocomposites (PNCs)

Polymer Nanocomposite (PNC) materials have been extensively documented in the scientific literature over the past few decades for their ability to significantly enhance properties, even with a low content of nanomaterials. In the field of nanotechnology, PNCs are characterized as solid materials composed of two or more distinct materials that are separated into phases, with one or more of the dispersed phases being at the nanoscale and a dominant polymeric phase. Materials are in the nanoscale range when at least one of their three external dimensions falls between approximately 1 nm and 100 nm. The addition of nanomaterials into the polymer matrices offers significant improvements in mechanical and other properties without compromising the parent properties and at a relatively low cost. Various organic and inorganic nanoparticles (NPs) have been documented in extensive prior literature as being utilized in the formulation of PNCs. Graphene, nanoclay, carbon nanotubes, carbon nanofibers, and metal oxide nanofillers have been extensively studied as reinforcing agents in PNCs for the FFF process.²⁸ These nanofillers have unique properties, such as low density, small dimensions, high aspect ratio, and superior mechanical, thermal, and electrical properties, making them suitable for a wide range of applications. However, the use of inorganic materials as fillers in the polymer matrix is gaining popularity due to their exceptional properties. The combination of inorganic materials (metal oxides NPs) properties, such as high hardness, high refractive index, thermal stability, and chemical stability, along with the desirable properties of organic polymers offers significant potential for a diverse range of applications.²⁹ The applications of these inorganic-organic PNCs span a wide spectrum, ranging from reinforcing in polymers to creating abrasion-resistant coatings, optical devices, flame-retardant materials, automotive components, fuel cells, photocatalysts, electrical insulation in electronics, and many more. The review article presents the research gaps in the AM through literature survey and provides the potential solution. The article summarizes the influence of reinforcing metal oxide NPs in material development, that lead to improve the material properties. Also, this review will discuss the challenges associated with the development of PNCs and provide a solution to improve the mechanical and weathering properties along with highlighting their potential applications.

Metal oxide-based polymer nanocomposites (MO-PNCs)

This section provides a concise review of the studies related to the MO-PNCs that enhance the mechanical and other properties of various polymer matrices across different manufacturing processes. The properties of the polymers used in different manufacturing processes can be improved by adding different metal oxide NPs. Metal oxide nanofillers, such as Titanium dioxide (TiO₂), Zinc oxide (ZnO), and Nanosilica (SiO₂), are extensively used in the literature in both scientific literature and commercial applications.³⁰ Those metal oxide nanofillers exhibit different characteristics, including UV resistance, oxygen and water barrier capabilities, chemical and thermal stability, mechanical strength, and customized rheological behavior. They possess excellent dispersion, fine nanoparticle size, high chemical purity, and a high specific surface area. These important characteristics make them a suitable choice for the development of PNCs for outdoor application fields.

Studies on nanocomposite with reinforcement of titanium dioxide in polymer matrix

Titanium dioxide (TiO₂) has garnered significant interest due to its non-toxic nature, excellent dispersibility, biocompatibility, and chemical stability, which enhance the properties of polymers. It has been extensively studied for various purposes, such as improving the UV stability and antibacterial properties of polymers. These characteristics have made it a preferred choice in various outdoor applications. The incorporation of TiO₂ NPs into polymer matrices has shown significant effects on the material properties and functionalities of various materials processed through different manufacturing techniques, including FFF/FDM, Injection Molding (IM), Extrusion, Coating, and Polymerization processes.

Using the FFF process, Vidakis et al.³¹ studied the effect of varying TiO₂ content (0–4 wt%) on the properties of PP polymer material. The results showed that the lower concentration mainly at 2 wt% of TiO₂ filler enhanced the mechanical properties, shown also in Figure 3 that improvement in strengths at different concentrations. The study demonstrated the potential of PP/TiO₂ nanocomposites (NCs) for FFF, offering improved mechanical properties without a significant impact on processability. In another study by Vidakis et al.³² developed a novel NCs filament produced using a melt mixing process by incorporating TiO₂ and antimony-doped tin oxide nanoparticles (ATO) into an ABS polymer matrix intended for use in the FFF process. The study found that the addition of TiO₂ and ATO fillers led to improvements in the overall tensile strength (9% $↑$ ), flexural strength (13% $↑$ ), and micro-hardness (6% $↑$ ) of nanocomposites compared to unfilled ABS. It showed the feasibility of fabricating physically and mechanically improved NCs materials suitable for commercial and industrial applications. Olam et al.³³ studied the effect of the printing temperature on PLA/TiO₂ NCs mixed with synthetic hydroxyapatite (HA), and natural hydroxyapatite (nHA) respectively. The addition of TiO₂ (1 wt%) and nHA (1 wt%) shows improved mechanical properties with better dimensional stability in compared to HA (1 wt%). In another study by Olam³⁴ reported that inclusion of the 4 wt% TiO₂ and effect of the printing parameters shows the significant enhancement in the UTS (36 $↑$ ), modulus of elasticity (26 $↑$ ). Soundararajan et al.³⁵ developed an polyamide (PA6) blends varying filler ratio of TiO₂ NPs. These composite blends build samples using FFF and were performed mechanical and tribological tests. The results showed that the significantly increased the tensile strength and Young’s modulus achieved with 30 wt% TiO₂, while elongation decreased. Tribological test results show improved wear resistance and sliding properties composite, will be the potential applications in the automotive industry. Kumar et al.³⁶ developed an multi-material matrix 3D printed biomedical scaffolds using the combination of the PLA and PA6/TiO₂ for biomedical applications. The antibacterial properties of PA6/TiO₂ combined with PLA to develop a feedstock filament using a twin-screw extrusion (TSE) machine and filament is then utilized in FDM to prepare specimens. The study identifies optimized FDM printing parameters for achieving superior flexural strength and demonstrates that the PA6/TiO₂ composite exhibits a lower wear rate compared to PLA.

Figure 3.

Summary of the mechanical properties of PP/TiO₂ nanocomposite materials.³¹

Petousis et al.³⁷ conducted a study aiming to optimize the mechanical response and antibacterial properties of PA12/TiO₂ NCs for AM applications. The addition of filler slightly reduced the mechanical strength but maintained sufficient strength for practical use. Moreover, the filler enhanced the antibacterial properties of the nanocomposites, making them suitable for medical applications. Low filler concentrations (2 wt%) achieved antibacterial performance without significantly increasing processing difficulty or cost. Kim et al.³⁸ developed 3D-printed structure of artificial cancellous bone of PLA/PCL/TiO₂ composite using FDM. The addition of TiO₂ NPs in PLA/PCL matrix enhanced the tensile strength up to 23% and in vitro biocompatibility shows that NPs helps in vigorous cell growth and properties enhancement. Brounstein et al.³⁹ focused on the development of antimicrobial composite filaments for 3D printing, utilizing PLA as the matrix material. The composite filaments incorporated ZnO and TiO₂ ceramics up to 30 wt% to confer antimicrobial properties. To adjust the mechanical properties without compromising antimicrobial efficacy, PEG was added as a plasticizer. The inclusion of ceramics enhanced the thermal stability of the filaments, with TiO₂ demonstrating superior stability compared to ZnO. The mechanical properties of the filaments could be tailored by adjusting the filler loading and type, as well as incorporating PEG. The ceramic fillers mitigated pitting and degradation caused by microbial activity, thereby providing antimicrobial properties to the PLA filaments. In the study by Singh et al.⁴⁰ focuses on controlling process parameters in twin screw extrusion to prepare reinforced PA6-TiO₂ hybrid feedstock filament (HFSF) for FDM. The study includes the potential for achieving desired mechanical properties with the recommended TSE process settings, the reduction in tensile properties due to TiO₂ aggregation on the PA6 fiber surface (which can be beneficial for biocompatibility and biodegradability in 3D printing), and the identification of the best input parameter settings for optimal mechanical properties of HFSF. In extrusion processes, the addition of TiO₂ nanoparticles to PLA not only enhances biodegradation rates, as noted by Mohr et al.,⁴¹ but also positively affects the composite’s overall mechanical properties. In melt extrusion processes involving PP studied by Alvardo et al.,⁴² the addition of TiO₂ NPs alongside CNT slightly decreased mechanical properties but improved electrical conductivity and reduced crystallinity and thermo-oxidative degradation, showcasing a trade-off between mechanical and electrical enhancements.

Photoactive TiO₂ NPs also been used in the development of the photocatalytic filters, as reported by Sangiorgi et al.⁴³ This study developed eco-friendly and biopolymer-based composites that is, 3D-printed TiO₂/PLA scaffolds using FDM. Surface modified TiO₂ NPs (30 wt%) optimized their dispersion and stabilization within the PLA solution, ensuring optimal distribution of the nano-photoactive points in the TiO₂/PLA filaments and scaffolds (as shown in Figure 4). This processing approach improved the dispersion of nano-charges compared to traditional methods, avoiding thermal degradation of the polymer, and allowing for customized product manufacturing. The study evaluated the photodegradation efficiency with varying TiO₂ content and filter geometry, demonstrating the potential of eco-friendly and biopolymer-based composites for 3D printing of photocatalytic filters with enhanced performance. In another study by Sevastaki et al.⁴⁴ built 3D photocatalytic structures using TiO₂/PS NCs filaments, containing up to 40%w/w of TiO₂ NPs through FDM. Notably, this work marks the first report of 3D-printed photocatalytic devices made entirely from recycled raw materials, with a high loading of TiO₂ NPs. The 3D-printed TiO₂/PS nanocomposite samples demonstrate excellent photocatalytic performance, achieving nearly 60% efficiency after multiple cycles of reuse. These structures exhibit efficient degradation of acetaminophen (APAP), indicating their potential as photocatalysts. The study showcases the feasibility of using TiO₂/PS NCs filaments for 3D-printed functional photocatalytic structures, opening new avenues in the field of photocatalysis. Yang et al.⁴⁵ developed a 3D-printed ceramic photocatalytic module through FFF and incorporating Ag/AgCl/TiO₂ as the photocatalyst in HDPE. The photocatalytic ability of the module to degrade MB dye and disinfect E. coli under both ultraviolet and visible light irradiation was investigated. The results showed that the module was effective in degrading MB dye and disinfecting E. coli within a short period, ranging from minutes to hours, depending on the specific surface area of the module. Even after undergoing five cycles of repeated use, the module maintained a high degradation efficiency of 95% for both dyes and bacteria. The photocatalytic module offers great potential as a reliable and structurally durable photocatalyst for degrading pollutants, presenting broad application prospects in pollutant treatment. Photocatalytic processes with TiO₂ have been particularly effective in improving the properties of various polymer matrices. Mekonnen et al.⁴⁶ further reported enhanced photocatalytic characteristics in conductive polymers, and Liu et al.⁴⁷ observed increased photocatalytic activity and wetting properties in polydimethylsiloxane (PDMS) composites.

Figure 4.

TiO₂ (30 wt%)/PLA photocatalytic filters 3D printed scaffolds.⁴³

Additionally, Martina et al.⁴⁸ reported that TiO₂ improves UV protection, increases transparency, and enhances the glass transition temperature in polyacrylate coatings through ex-situ and in-situ polymerization. In in-situ emulsion polymerization, Xiang et al.⁴⁹ found that TiO₂ significantly improves impact toughness and solar reflectance in acrylonitrile-styrene acrylate (ASA) composites. Wang et al.⁵⁰ also reported that mini-emulsion polymerization with TiO₂ leads to enhanced thermal properties and excellent UV shielding capacity in polyacrylate composites. Furthermore, in injection molding, Deniz et al.⁵¹ found that increasing the amount of nano-TiO₂ in PP matrices results in improved mechanical properties, except for tensile modulus and impact strength, while also reducing water absorption and enhancing thermal stability. Overall, the incorporation of TiO₂ NPs to polymer matrices across various manufacturing processes consistently enhances mechanical properties (tensile strength, flexural strength, and wear resistance), also improves thermal stability, UV protection, and photocatalytic activity, contributing to the development of more durable, efficient, and environmental friendly materials. These advantages make TiO₂ a valuable nanomaterial in the development of advanced polymer composites, offering a versatile approach to improving material performance in both structural and functional applications. Several research studies have investigated the use of TiO₂ as a filler in polymer matrices to develop nanocomposites, which has been summarised in Table 3.

Table 3.

Summary of studies on composites with reinforcement of titanium dioxide in polymer matrix.

Author (Year)	Polymer matrix	Nanomaterial	Process	Effect on mechanical and weathering properties	Ref
Olam (2022)	PLA	1% HA and 4% TiO₂	FDM	- Addition of nanofillers enhances UTS and modulus of elasticity	³⁴
Vidakis et al. (2021)	PP	TiO₂ NPs	FFF	- Mechanical properties were improved compared to pure PP polymer	³¹
				- Processability was not significantly affected, even at high loadings
Brounstein et al. (2021)	PLA	ZnO and TiO₂	FFF	-30–50% stress reduction, decrease in Young’s modulus, and increase in elongation	³⁹
				- Significantly reduces pitting and degradation
Olam et al. (2021)	PLA	TiO₂, HA and nHA	FDM	- No significant change in the dimensional analysis of 3D printed samples	³³
				- Composite mixture of nHA shows better mechanical properties than HA
Sevastaki et al. (2020)	PS	TiO₂ NPs	FDM	- Offer promising photocatalytic properties	⁴⁴
Vidakis et al. (2020)	ABS	TiO₂, ATO	FFF	- Enhanced tensile strength by 7%, flexure strength by 12%, and micro-hardness by 6%	³²
				- For ATO filler, the increases were 9%, 13%, and 6% respectively
Kumar et al. (2020)	PLA-PA6	TiO₂	FDM	- Enhanced flexural and wear properties of PLA with PA6/TiO2	³⁶
Sangiorgi et al. (2019)	PLA	TiO₂ NPs	FDM	- Photocatalytic filters	⁴³
Soundararajan et al. (2019)	PA6	TiO₂	FDM	- Higher mechanical properties and minimum wear rate at 30 wt% TiO2	³⁵
Kim et al. (2018)	PLA/PCL	1 wt% TiO₂	FDM	- Potential enhancement in biocompatibility and degradability	³⁸
				- Increased tensile strength
Singh et al. (2018)	PA6	TiO₂	FDM	- Enhanced the biocompatibility and biodegradability	⁴⁰
Alvarado et al. (2019)	PP	CNT, TiO₂ NPs	Melt extrusion process	- Decreased crystallinity and thermo-oxidative degradation	⁴²
				- Slight decrease in mechanical properties
				- Improved electrical conductivity
Mohr et al. (2019)	PLA	TiO₂ NPs	Extrusion	- Enhanced biodegradation rate	⁴¹
Yanli et al. (2017)	ASA	TiO₂ NPs	Melt blending	- Significant improvement in solar reflectance and cooling properties	⁵²
				- Increased water contact angle
Martina et al. (2021)	PA	TiO₂	Ex-situ and In-situ polymerization, coating	- Better dispersibility of TiO2 NPs, improved UV protection, and greater transparency of the PA coating films	⁴⁸
				- Increased glass transition temperature
Wang et al. (2016)	Polyacrylate	TiO₂ NPs	Miniemulsion polymerization	- Significantly enhanced thermal properties	⁵⁰
				- Excellent UV shielding capacity with high visible transparency
Xiang et al. (2016)	ASA	TiO₂	In situ emulsion polymerization	- Significant improvement in impact toughness	⁴⁹
				- Better dispersibility and higher solar reflectance
Deniz et al. (2016)	PP	TiO₂ NPs	Injection molding	- Increased mechanical properties with increased nano-TiO2, except for tensile modulus and impact strength	⁵¹
				- Decreased water absorption and increased thermal stability
Mekonnen et al. (2021)	Conductive Polymer	TiO₂ NPs	Photocatalysis	- Enhanced photocatalytic characteristics	⁴⁶
Liu et al. (2019)	PDMS	TiO₂	Photocatalysis	- Increased photocatalytic activity and wetting properties	⁴⁷

Studies on nanocomposite with reinforcement of zinc oxide in polymer matrix

Zinc oxide (ZnO) is a promising nanocomposite known for its multifunctional properties, including high chemical stability, a high refractive index, thermal conductivity, antibacterial and UV protection capabilities, and environmental friendliness. It serves as an ideal filler for high-performance polymer/inorganic nanocomposites for used in outdoor fields. Additionally, ZnO NPs possess desirable attributes such as biocompatibility, non-toxicity, and cost-effectiveness, making them a preferable option for researchers. Zinc oxide (ZnO) has been widely studied as a reinforcing nanomaterial for various polymer matrices, showing significant effects on material properties across different manufacturing processes.

In FFF, Joshi et al.⁵³ investigated the impact of ZnO NPs and processing conditions on crystallinity, viscosity, and electromechanical properties of PVDF and PVDF/ZnO NCs. The addition of ZnO to the PVDF resulted in improved thermal stability and an increase in the glass transition temperature and decrease shear viscosity. 3D printed samples showed higher elastic modulus and toughness. The strength of printed parts was influenced by the infill direction, and FTIR confirmed the promotion of the β-phase with specific ZnO percentages. High cooling rates combined with 7.5-12.5% ZnO content resulted in up to 80% conversion of PVDF from the α-phase to the β-phase, resulting in a 40% increase in elastic modulus while maintaining ultimate strength. These findings have significant implications for the development of advanced materials to develop piezoelectric biomedical sensor using FFF. Similarly, Kumar et al.⁵⁴ developed an ZnO reinforced PVDF composites for biosensor 3D printing. The addition of 1%wt ZnO NPs decreased the mechanical properties, while 2%wt ZnO improved them but remained lower than PVDF without ZnO. Increasing the ZnO percentage improved thermal stability and increased the normalized heat capacity. PVDF-2%ZnO demonstrated a shape memory effect of 98.22% at 25°C. Based on the findings, the study recommended an optimal composition of 1% ZnO, specific forced loading, and torque for achieving maximum mechanical strength. The PVDF-ZnO composites exhibited hydrophobic behavior suitable for biosensor 3D printing. Additionally, the composites showed excellent shape recovery and acceptable printability. In another study of Kumar et al.⁵⁵ investigated the manganese-doped ZnO (Mn-ZnO) NPs reinforced to PVDF for improving the composite properties. The pre-heat treatment step improves the mechanical strength of the composites by releasing gases and facilitating particle settlement. Higher barrel temperature, increased particle concentration, and lower screw torque enhance blending and improve the mechanical properties. The feedstock filament with the highest tensile strength exhibits a value of 45.89 MPa. Moreover, the PVDF-3%(Mn-ZnO) composite samples exhibit magnetic, piezoelectric, and shape memory properties, making them suitable for sensor technologies and indicating their potential for 4D printing applications. Singh et al.⁵⁶ investigated the two-way programmed shape memory properties of ZnO-reinforced PLA prototypes fabricated using FDM. The results showed that specific combinations of infill density, the number of perimeters, and the pattern significantly influenced the tensile properties of the prototypes. The porosity of the prototypes responded to changes in water temperature, increasing to 70°C and decreasing to 10°C, but returning to the original state at atmospheric temperature. The study also observed shape memory effects on volume and weight at different temperatures. Overall, the findings emphasize the importance of selecting suitable thermoplastic matrices for structural applications exposed to varying water temperatures.

The study conducted by Vidakis et al.⁵⁷ focuses on the production of nanocomposite and microcomposite filaments through melt extrusion for 3D printing applications. ZnO NPs (ZnO nano) and ZnO microparticles (ZnO micro) are dispersed in ABS matrices at various concentrations. The results indicate improved tensile and flexural strength in both nanocomposite and microcomposite materials compared to pure ABS. ZnO NPs has been effective in increasing the tensile and flexural strengths in ABS composites, reported that 14% and 15.3% improvement, respectively. Lee et al.⁵⁸ successfully fabricated hierarchical structures of ABS-ZnO NCs on a 3D-printed backbone using FDM and hydrothermal reaction techniques. The photocatalytic performance of these structures was evaluated for the degradation of methylene blue, an organic pollutant. The aim was to provide an effective solution for environmental challenges using photocatalysis. Three hierarchical structures, namely NPs, nanorods (NRs), and nanoflowers (NFs), were created and compared. Among these structures, the ZnO-NFs on the 3D-printed backbone exhibited the highest efficiency in decomposing the organic solvent due to their larger active surface area. The study demonstrated the potential of using ZnO-based hierarchical structures on 3D-printed backbones as sustainable and efficient photocatalysts to address environmental issues (as shown in Figure 5).

Figure 5.

Schematic of the ZnO-based hierarchical structures on the 3D-backbone. (a) Schematic illustration of ZnO-NPs, NRs, and NFs on the 3D-backbone, and (b) photographs of 3D-backbone (ABS + ZnO) and ZnO-NFs hierarchical structure on the 3D-backbone.⁵⁸

In coating processes, ZnO has been effectively used to enhance various properties. Ariffin et al.⁵⁹ developed a bio-based polyurethane acrylate (PUA) coatings with nano-ZnO exhibited enhanced anticorrosive properties, and polyurethane coatings showed slight increases in hydrophilicity and substantial improvements in corrosion resistance. Xiaoyun et al.⁶⁰ reported that ZnO coatings on polyurethane (PU) via spin coating resulted in enhanced mechanical properties, including corrosion resistance, while maintaining slight hydrophilicity. Moreover, Wasim et al.⁶¹ found that sputter-coated bacterial cellulose with copper and ZnO provided excellent UV resistance, anti-static properties, and antibacterial characteristics. In-situ and ex-situ polymerization processes have also shown favourable results with ZnO. Changpo et al.⁶² revealed that the in-situ emulsion polymerization of polyacrylate with silicon-modified ZnO significantly increased the thermal degradation temperature and provided self-cleaning and oil-water separation properties. Additionally, Mohammad Rahman⁶³ noted that ZnO incorporated via in-situ polymerization, blending, and coating processes in polyurethane enhanced protective properties, including adhesive strength and self-healing capabilities. In other manufacturing processes, such as radiation-induced graft polymerization and physical magnetron sputtering, ZnO has been found to improve the thermal resistance, UV resistance, and durability of materials, as demonstrated by Wang et al.⁶⁴ in PET fabric and Wang et al.⁶⁵ (2021) in wood. Hand lay-up methods have also benefited from ZnO, as Mohammed et al.⁶⁶ observed increased flexural strength stability, break elongation, weathering stability, and moisture resistance in PE/kenaf composites. The inclusion of ZnO NPs in polymer matrices across various manufacturing processes offers numerous advantages, including improved mechanical properties such as increased tensile strength, modulus, and toughness. While also imparting functional benefits, including enhanced thermal, moisture, and UV resistance, and superior protective features such as corrosion resistance and antibacterial activity. These enhancements make ZnO a valuable additive in various polymer matrices across different manufacturing processes, contributing to the development of high-performance, durable, and multifunctional materials for outdoor fields. Several research studies have investigated the use of ZnO as a filler in polymer matrices to develop nanocomposites, which has been summarised in Table 4.

Table 4.

Summary of studies on composites with reinforcement of zinc oxide in polymer matrix.

Author (Year)	Polymer matrix	Nanomaterial	Process	Effect on mechanical and weathering properties	Ref
Joshi et al. (2022)	PVDF	Zinc oxide (ZnO)	FFF	- Increased the glass transition temperature with increasing the ZnO content	⁵³
Joshi et al. (2022)	PVDF	Zinc oxide (ZnO)	FFF	−40% increase of the elastic modulus and increases toughness while maintaining the same ultimate strength of the pristine PVDF	⁵³
Kumar et al. (2021)	PVDF	Mn doped ZnO	FFF	- Reinforcement of Mn-ZnO in PVDF enhances piezoelectric, magnetic, and shape memory properties	⁵⁵
Kumar et al. (2020)	PVDF	ZnO NPs	FDM	−2%ZnO-PVDF composite showed better performance than PVDF-1%ZnO	⁵⁴
Kumar et al. (2020)	PVDF	ZnO NPs	FDM	- ZnO NPs addition enhances normalized heat capacity	⁵⁴
Vidakis et al. (2020)	ABS	ZnO NPs and ZnO MPs	FFF	- For ABS/ZnO nanocomposites, 14% increase in the tensile strength at 5% wt. and 15.3% increase in the flexural strength was found in 0.5% wt	⁵⁷
Vidakis et al. (2020)	ABS	ZnO NPs and ZnO MPs	FFF	- Similar enhancements in ABS/ZnO micro composites, 14% and 17% increase in the tensile and flexural strength respectively at 5% wt	⁵⁷
Singh et al. (2020)	PLA	ZnO	FDM	- Enhances the mechanical properties and porosity value decreases	⁵⁶
Lee et al. (2018)	ABS	ZnO NPs	FDM	- Enhances the photocatalytic performance	⁵⁸
Wasim et al. (2020)	Bacterial Cellulose	Copper and Zinc oxide	Sputter coating	- Good ultraviolet resistance, anti-static and antibacterial characteristics	⁶¹
Ariffin et al. (2020)	Bio-based PUA	Nano ZnO	Coating	- Improved anticorrosive properties	⁵⁹
Xiaoyun et al. (2019)	PU	Zinc oxide (ZnO)	Coating	- Slight increase in hydrophilicity and improved mechanical properties with increased ZnO content	⁶⁰
Xiaoyun et al. (2019)	PU	Zinc oxide (ZnO)	Coating	- Exhibited excellent corrosion resistance	⁶⁰
Changpo et al. (2021)	Polyacrylate	Si modified ZnO	In situ emulsion polymerization	- Thermal degradation temperature of the composites increased significantly	⁶²
Changpo et al. (2021)	Polyacrylate	Si modified ZnO	In situ emulsion polymerization	- Coating has excellent self-cleaning and oil– water separation properties	⁶²
Mohammad Rahman (2020)	PU	ZnO	In situ Polymerization, blending, coating	- Enhanced protective properties, including adhesive strength and self-healing	⁶³
Shiv Shankar et al. (2019)	PLA/PBAT	ZnO NPs	Solution Casting method	- Exhibited strong ultraviolet shielding properties	⁶⁷
				- No significant changes in surface color, transparency, or chemical structure
				- Significant increase in tensile strength and strong antibacterial activity
Wang et al. (2019)	PET fabric	ZnO NPs	Radiation induced graft polymerization	- Exhibited excellent thermal resistance, UV resistance and durability	⁶⁴
Mohammed et al. (2018)	PE/kenaf composites	ZnO NPs	Hand lay-up method	- Increased flexural strength stability and break elongation	⁶⁶
Mohammed et al. (2018)	PE/kenaf composites	ZnO NPs	Hand lay-up method	- Increase weathering stability highest moisture resistance	⁶⁶

Studies on nanocomposite with reinforcement of nanosilica in polymer matrix

Nanosilica (SiO₂) is a highly effective inorganic filler for PNCs, offering several advantages over other NPs, including UV filtration, cost-effectiveness, non-toxicity, biocompatibility, low refractive index, and high thermal and mechanical stability. Its incorporation improves the processability of polymers during various manufacturing methods. This section summarizes studies on the reinforcement of nanosilica in polymer matrices has showed significant enhancements in material properties across different manufacturing processes such as FFF/FDM, injection molding, compression molding, and coating techniques.

Vidakis et al.⁶⁸ conducted a study investigating the use of SiO₂ NPs as fillers in polypropylene (PP) for FFF 3D printing. The addition of the SiO₂ NPs enhanced the mechanical properties of the PP matrix without hindering processability, particularly at 1.0–2.0 wt% loadings. The results demonstrated that the addition of SiO₂ NPs improved the mechanical performance of the PP matrix, with the best results achieved at a loading of 1.0 wt%. The impact strength and microhardness were marginally affected. Dynamic mechanical analysis (DMA) revealed a stiffer behavior for PP/SiO₂ 1.0 wt% nanocomposites, indicating enhanced overall mechanical response. SEM confirmed the quality of the specimens and the manufacturing process but revealed defects and discontinuities in interlayer fusion, which could impact mechanical properties. Fracture surface analysis showed visible gaps between strands in 3D printed specimens, with worsened fusion at higher filler loadings. Overall, incorporating SiO₂ in PP shows promise in enhancing mechanical and thermomechanical properties and countering wrapping effects during 3D printing. Vidakis et al.⁶⁹ conducted a another study to develop novel nanocomposite filaments for 3D printing using PLA and SiO₂ NPs. Different concentrations (0.5, 1.0, 2.0 and 4.0 wt%) of SiO₂ were incorporated into the PLA matrix, and the resulting filaments were characterized and tested. Tensile, flexural, and impact tests showed that the addition of SiO₂ enhanced the mechanical properties, with the highest improvement at a concentration of 1.0 wt% (Figure 6). However, concentrations above 1.0 wt% resulted in decreased strength, but at 4.0 wt%, the NC exhibited mild antibacterial properties, particularly against S. aureus, suggesting potential for antibacterial applications in 3D printing.

Figure 6.

Summary of the mechanical properties of PLA/SiO₂ nanocomposite materials.⁶⁹

The study conducted by Street et al.⁷⁰ investigates the utilization of silica NPs to enhance the mechanical properties of FFF-printed PMMA parts. Through the incorporation of silica NPs in PMMA matrix, notable improvements were observed in thermomechanical properties, including glass transition temperature, tensile strength, modulus, and elongation. The presence of Silica NPs led to the development of hierarchical structures and enhanced hydrogen bonding interactions between the PMMA polymer matrix and the surface of the silica NPs. This facilitated stress transfer and restricted chain mobility, resulting in improved material properties. The findings suggest that the incorporation of inorganic NPs into nanocomposite filaments can overcome the limitations of FFF-printed parts and combine the desirable characteristics of both hard and soft matter. Overall study contributes to the utilizing NCs to achieve a wider range of material properties and address the limitations of FFF-printed parts. Lin et al.⁷¹ conducted study that focuses on the preparation of a carbon fiber (CF) reinforced PEEK coating material using FDM. The tribological properties of the coating, with and without nanosilica at the interface, were investigated. The orientation of the sliding direction relative to the CF strongly influenced the friction coefficient and wear rate. The addition of nanosilica at the interface improved the friction performance by reducing adhesion and introducing a rolling effect. The study suggests that external incorporation of NPs at the sliding interface can reduce the friction coefficient, while internal inclusion in the coating material may improve wear resistance. These findings demonstrate the potential for enhancing the tribological performance of PEEK/CF composite coatings in manufacturing high-performance tribo-composites with complex geometries. The study conducted by Kyratsis et al.⁷² focuses on enhancing the capabilities of 3D printed structures by incorporating nanosilica particles into ABS polymer. The results indicated that the addition of nanosilica increased the stiffness and hardness of the ABS material, particularly at a concentration of 10%wt. However, 3D printing was found to be unfeasible at this concentration due to material embrittlement. Tensile specimens printed from the 5%wt nanocomposites exhibited higher breaking point stress and stiffness compared to pure ABS. The morphology of the filaments showed roughness and areas of agglomeration attributed to the dispersion of nanosilica.

Lyu et al.⁷³ presents a green chemical method for coating poly (D-lactic acid) (PDLA) onto SiO₂ NPs to modify the performance in polymer composites. The coated nanoparticles enable nano dispersion in a biodegradable blend of poly (L-lactic acid)/poly (butylene adipate terephthalate) (PLLA/PBAT) and uniform distribution of stereocomplex crystallites (SC) in the matrix. The mechanical properties of the composites are influenced by the crystallization behavior, which can be controlled by adjusting the 3D printing nozzle temperature to achieve a “strength-toughness” transition. The modified SiO₂ exhibits good dispersion even at high loading, indicating good compatibility and preventing agglomeration. The addition of SiO₂-PDLA improves heat resistance due to the presence of SC. At a lower nozzle temperature, the composites show enhanced tensile strength, while at a higher temperature, the SC melts, affecting the mechanical properties. The grafting of PDLA acts as a plasticizer, enhancing toughness. The addition of silica as a filler in polymeric waste from 3D printing processes, and its effect on the tensile strength of polymers such as PLA, PVA, and nylon, has been investigated by Ahmed et al.⁷⁴ However, increasing the silica content decreases the toughness and ductility of the materials. The modulus of elasticity and yield strength also decreased, except for PLA. The density of silica/polymer mixtures generally decreases with higher silica percentages, except for PP, which shows a slight increase. Overall, studying the properties of recycled polymeric waste and its incorporation into 3D printing contributes to sustainable practices and the utilization of waste materials. The research conducted by Ramachandran et al.⁷⁵ focused on the development of PLA/nano-silica bio-nanocomposite filaments for 3D printing using the FDM process. The addition of nano-silica had a significant positive impact on the mechanical and tribological properties of the printed parts, owing to its uniform dispersion in the PLA matrix. Specifically, the PLA/nano-silica filament with 8 wt% nano-silica exhibited superior performance over 0 to 6 wt% concentrations. The highest values for tensile, compression, flexural, impact strength, and hardness were achieved with an 8 wt% nano-silica concentration. Furthermore, the inclusion of 8 wt% nano-silica led to a reduced friction coefficient and specific wear rate, indicating improved tribological properties. Al-Marzouqi et al.⁷⁶ investigate the mechanical properties of ABS matrix composites reinforced with nanosilica particles, which prepared using hot extrusion process. The results show that increasing the nanosilica content reduces the tensile strength but improves the toughness, ductility, and yield stress of the ABS/nanosilica composites. The research suggests further exploration of combining silica with waste ABS for 3D printing and investigating the mechanochemical stability of the final product. Additionally, the combination of silica with biomass could lead to the development of hybrid biodegradable composite materials via 3D printing.

Nanosilica has also been effectively utilized in other manufacturing processes. In coating applications, Thien et al.⁷⁷ highlighted the dual role of SiO₂ nanoparticles as a reinforcer and UV absorber in acrylic polyurethane, enhancing mechanical properties, weathering, and abrasion resistance at lower concentrations. Fuying et al.⁷⁷ reported that nanosilica coatings on fluorinated polyacrylate polyurethane exhibited good hydrophobicity, anti-aging properties, improved chemical resistance, and mechanical performance, with less than 30% gloss loss after extended UV exposure. Additionally, Zahid et al.⁷⁸ noted increased hydrophobicity and current density in PMMA after silica nanoparticle coating via spin coating. In injection molding, Titone et al.⁷⁹ demonstrated that SiO₂ significantly improved the thermomechanical properties, elastic modulus, tensile strength, photostability, and oxygen barrier properties of polypropylene (PP) composites. Compression molding studies by Maria et al.⁸⁰ and Fatemeh et al.⁸¹ also showed improved mechanical and wear properties, good nanosilica dispersion, and enhanced thermal stability in polyamide (PA) and PLA/PC blend composites, respectively. The incorporation of nanosilica into polymer matrices across various manufacturing processes results in enhanced mechanical properties, such as tensile strength, flexural strength, stiffness, impact and wear resistance. It also improves thermal stability, as well as weathering resistance, including UV protection, and hydrophobicity. These benefits make nanosilica an attractive additive for the development of PNC materials, paving the way for their application in various high-performance and environmentally challenging scenarios. Several research studies have investigated the use of SiO₂ as a filler in polymer matrices to develop nanocomposites, which has been summarised in Table 5.

Table 5.

Summary of studies on composites with reinforcement of nanosilica in polymer matrix.

Author (Year)	Polymer matrix	Nanomaterial	Process	Effect on mechanical and weathering properties	Ref
Al-Mazrouei et al. (2022)	ABS	Silicon Dioxide (SiO₂) - Micro particulate	FFF	- Increased toughness, ductility, and yield stress at 15 wt% silica	⁷⁶
Al-Mazrouei et al. (2022)	ABS	Silicon Dioxide (SiO₂) - Micro particulate	FFF	- Reduced tensile strength to 22.4 MPa at 10 wt%	⁷⁶
Lyu et al. (2022)	PDLA	SiO₂ NPs	FDM	- Enhanced heat resistance	⁷³
Lyu et al. (2022)	PDLA	SiO₂ NPs	FDM	- Improved the tensile strength with SC and SiO₂-PDLA distribution	⁷³
Vidakis et al. (2021)	PP	SiO₂ (0.5, 1.0, 2.0, 4.0 wt%)	FFF	- Enhanced tensile, flexural and impact properties of composite up to 2% wt SiO₂	⁶⁸
Vidakis et al. (2021)	PLA	SiO₂ (0.5, 1.0, 2.0, 4.0 wt%)	FFF	- Increased the tensile, flexural and impact strength at 1% wt SiO₂	⁶⁹
Vidakis et al. (2021)	PLA	SiO₂ (0.5, 1.0, 2.0, 4.0 wt%)	FFF	- Mild antibacterial properties at 4.0 wt%	⁶⁹
Rasana et al. (2021)	ABS	MWCNT 5% (wt) and 3% (wt) Nanosilica	FFF	- Enhanced stiffness and tensile strength	⁸²
Rasana et al. (2021)	ABS	MWCNT 5% (wt) and 3% (wt) Nanosilica	FFF	- Improved reinforcing efficiency and entanglement density	⁸²
Ahmed et al. (2021)	PP, PVA, PLA, and Nylon	Nanosilica (5, 10, and 15 wt%)	FFF	- Improved rheological properties and tensile strength	⁷⁴
Ramchandran et al. (2021)	PLA	Nanosilica (0, 2, 4, 6, 8 wt%)	FDM	- Optimal mechanical properties at 8 wt%	⁷⁵
Ramchandran et al. (2021)	PLA	Nanosilica (0, 2, 4, 6, 8 wt%)	FDM	- Reduced friction coefficient and specific wear rate	⁷⁵
Seng et al. (2020)	PLA	Nanosilica (0, 1, 3 wt%)	FDM	- Reduced hygroscopicity by 40% at 1% wt	⁸³
Seng et al. (2020)	PLA	Nanosilica (0, 1, 3 wt%)	FDM	- Higher the tensile strength, Young’s modulus, and elongation at 3% wt	⁸³
Lin et al. (2019)	PEEK/CF Composite	Nanosilica	FDM	- Significantly improved friction performance	⁷¹
Huang et al. (2019)	ABS	Cellulose nanocrystals/silica nanohybrids (CSNs)	FDM	- Enhanced layer adhesion and mechanical properties	⁸⁴
Huang et al. (2019)	ABS	Cellulose nanocrystals/silica nanohybrids (CSNs)	FDM	- Applicability to high-temperature applications	⁸⁴
Kyratsis et al. (2018)	ABS	Nanosilica (5 & 10 wt%)	FFF	- Higher breaking point stress and stiffness	⁷²
Street et al. (2018)	PMMA	Silica NPs	FFF	- Increased glass transition temperature, Young’s modulus, elongation at break, and tensile strength	⁷⁰
Hong et al. (2017)	PLA	Nano silica /hydroxyapatite	FFF	- Retained 80% of mechanical strength after in-vitro degradation (12 weeks)	⁸⁵
Meng et al. (2016)	ABS	Nano silica	FDM	- Reduced mechanical anisotropy and improved mechanical strength and thermostability	⁸⁶
Zahid et al. (2021)	PMMA	Silica NPs	Spin coating	- Increased hydrophobicity and current density with silica nanoparticle coating	⁷⁸
Thien et al. (2020)	Acrylic Polyurethane	SiO₂ NPs	Coating	- Dual role as reinforcer and UV absorber	⁷⁷
				- Enhanced mechanical properties and weathering resistance
				- Lower concentration (2 wt%) exhibits excellent weathering and abrasion resistances
Fuying et al. (2020)	Fluorinated polyacrylate polyurethane	Nano-SiO₂	Coating	- Coating that shows both good hydrophobicity and anti-aging properties	⁸⁷
				- Improved chemical resistance and mechanical properties
				- Coating as demonstrated by less than 30 % of gloss loss after an artificial UV irradiation for 720 h
Titone et al. (2021)	PP	SiO₂ (0, 1 and 2 wt/wt%)	IM	- Improved thermomechanical properties, elastic modulus, tensile strength	⁷⁹
Titone et al. (2021)	PP	SiO₂ (0, 1 and 2 wt/wt%)	IM	- Presence of silica improves the photostability (absorbance of the UV radiation) and barrier to the oxygen	⁷⁹
Maria et al. (2020)	PA	Nanosilica	Compression molding	- Improved mechanical and wear properties with 1% nanosilica	⁸⁰
Fatemeh et al. (2019)	PLA/PC blend	Silica NPs	Compression mold	- Good nanosilica dispersion and improved thermal stability	⁸¹

Challenges and opportunities of MO-PNCs

A variety of polymer materials have been processed through FFF and other manufacturing processes, but the limited properties of those materials cause a constraint on their ability to perform optimally under different environmental conditions. FFF relies on highly hygroscopic polymers such as ABS, PP, Polyamide (PA6, PA12), PLA, and HDPE, which exhibit weak durability when exposed to outdoor environmental conditions due to factors like sunlight, moisture, high temperature, and chemicals. Some of the thermoplastic polymers, such as PC, PET-G, PMMA, ASA, and PEEK, have been great alternatives for outdoor applications due to their better resistance to diverse environmental conditions. In recent years, significant research has been conducted on integrating nanofillers with polymers in FFF to enable broader applications of developed nanocomposite materials. The paper summarizes the discussion related to material development by reinforcing metal oxide NPs and their obtained properties. Although metal oxides NPs have exceptional properties, some disadvantages make them incompatible with polymer matrices. The addition of NPs may cause agglomeration formation, heterogeneous nanocomposite formation, non-adhesion, and non-uniform dispersion, which are the key challenges that restrict the ability to enhance the material properties. The agglomeration of NPs will reduce the interfacial area and the interactions with the polymers in nanocomposites, thereby negating the potential benefits of using NPs. In some cases, agglomeration may even result in the deterioration of material properties and act as a defect in the structures. To overcome this issue, a suitable preparation strategy needs to be identified that results in the successful integration of NPs in polymer matrices. PNCs can be prepared using different approaches, such as in-situ polymerization and ex-situ polymerization (solution or melt blending). Regardless of the preparation approach, it is necessary to ensure good compatibility between the NPs and polymer matrix to achieve a homogeneous PNC. Compatibility can be improved by selecting appropriate blending conditions, such as the concentration of polymers and NPs, or by adjusting the temperature range and mixing speed. Also, the reinforcing effect of NPs is attributed to several factors, such as the type of nanofillers, particle aspect ratio, size, orientation, and distribution. To counteract the shortcomings of metal oxide NPs, a suitable preparation approach has been used to produce MO-PNCs with superior material performance.

Amidst these challenges, MO-PNCs offer promising avenues for the development of sustainable and weather-resistant 3D printing material. The incorporation of metal oxide NPs, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and nano-silica (SiO₂), has led to significant enhancements in material processability, as well as mechanical, tribological, thermal, photocatalytic, antibacterial, and biodegradability properties. The present study highlights relevant literature that explores the addition of these metal oxide NPs to polymer matrices to improve mechanical properties and weathering characteristics, including UV stability, thermal stability, chemical resistance, oxygen and water barrier properties, cooling properties, and anticorrosive properties. Therefore, these metal oxides, which have a hydrophobic nature and strong UV absorbance properties, can protect the polymer from the synergistic effects of various environmental conditions in outdoor fields. The diverse characteristics of MO-PNCs make them suitable for industrial applications requiring improved performance, durability, weather resistance, and ushering in a more sustainable and environmentally friendly future. By incorporating MO-PNCs, there is potential to open up new opportunities for creating high-performance FFF components, thereby expanding their suitability for outdoor applications (Figure 7). Opportunities for these MO-PNCs exist in various sectors, including automotive, electrical, aerospace and defense, medical and healthcare, marine, and other outdoor areas mentioned in Table 6.

Figure 7.

Application areas for the MO-PNCs material.

Table 6.

Applications areas of MO-PNCs.

Applications areas	Components
Automotive	- Engine components, side mirror casing, exhaust manifolds, gearbox housing, coolant tank, spare parts, dashboards, seat backs, and interior panels, jigs and fixtures, door handles
Electrical	- Electrical connectors, sockets, switches, protective casings for circuits, and antennas
Aerospace and Defense	- Engine brackets, dashboard parts, turbine blades, unmanned aerial vehicles (UAVs) parts, satellite brackets, panels, drone components
Medical and Healthcare	- Surgical instruments and device components
Construction and Infrastructure	- Insulation panels, facade elements, roofing materials, and wall claddings
Marine and other outdoor areas	- Sports equipment (helmets, bicycle frames and parts, toys), furniture (chair frames, decorative items), consumer goods (home appliances, and kitchenware)

Applications of MO-PNCs

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Sagar Kailas Gawali

Prashant K Jain

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.*

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Advancement of nanofiller reinforced fused filament fabrication polymers: A path to sustainable,weather-resistant 3D printing materials

Abstract

Keywords

Introduction

Additive manufacturing (AM) and polymer materials

Polymer nanocomposites (PNCs)

Metal oxide-based polymer nanocomposites (MO-PNCs)

Studies on nanocomposite with reinforcement of titanium dioxide in polymer matrix

Studies on nanocomposite with reinforcement of zinc oxide in polymer matrix

Studies on nanocomposite with reinforcement of nanosilica in polymer matrix

Challenges and opportunities of MO-PNCs

Applications of MO-PNCs

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Data availability statement

Appendix

References