Recycling polypropylene from non-woven disposable masks in developing a three-dimensional printing filament

Abstract

Non-woven disposable masks play a unique role in reducing the COVID-19 pandemic threat in transmission between people, but the huge amount of disposable non-woven masks generated every day are currently posing a serious challenge to our environment on a global-wide scale. In line with this emerging problem, a series of recycling processes were designed and conducted to evaluate the performance of material recovered from those waste masks for potential use in three-dimensional (3D) printing. A composite filament from recycled polypropylene (rPP) and an additive material, glass fiber (GF), was fabricated by melt-blending processing followed by single-screw extrusion. A variety of material properties, including the chemical/mechanical/microstructure property, thermal stability, printability, rheology performance, and geometrical accuracy toward GF/rPP composite filaments, were comprehensively analyzed. Our results demonstrated that two important mechanical properties, the compression strength and the tensile strength, to a 3D printed object by fused deposition modeling (FDM) from the GF/rPP composite were significantly higher than that of a FDM 3D printed object from GF/polypropylene composites. The specific warpage parameter (Wsp) and the surface roughness (Sa) for a 3D printed object from the GF/rPP composite at 30 wt% GF additive would have printing accuracy of 0.54% ± 0.0014 and 21.1 ± 0.76 µm, respectively, and no clogging phenomenon was observed in the printer nozzle channel during the printing processing, suggested that this recycling method for a large number of non-woven waste masks was potentially applicable in serving as a FDM 3D printing material.

Keywords

Non-woven mask textile wastes recycling fused deposition modeling three-dimensional printing melt-blending extrusion COVID-19

During the novel coronavirus (COVID-19) pandemic, wearing non-woven face masks is one of the viable proactive measures to significantly prevent the spread of COVID-19, which has inevitably led to an unprecedented growth in the global production of single-use non-woven masks made from polymeric materials.^1,2 It is estimated that people worldwide used up to 129 billion face masks every month in 2021, which amounted to an average of over 2.8 million masks used every minute. Meanwhile, based on an estimated weight of approximately 5 g for each mask, the world will face about 14,000 tons of waste masks every day.^3,4 Currently, COVID-19 seems to have no end in sight. According to epidemiologists, wearing single-use masks will become the norm for a long time to keep out bacterial and viral infections.⁵

Unfortunately, along with the significant increase in non-woven mask production and consumption worldwide, a new challenge for our environment and human health has been further put forward. The non-woven masks are made of non-biodegradable plastic, which means that they will remain in the environment for hundreds of years if they are not recycled.⁶ The used non-woven masks can be spotted almost everywhere, causing urban environmental chaos especially in some local parks, underground exits, and car parking lots due to an uptick in littering. Even though some countries requested citizens to dispose of masks in special places such as trash bins, or where the masks sit in landfill sites, due to the lightweight nature of the masks, wind and rainwater can move the masks freely into city streets or rivers and oceans. Most of the non-woven masks pouring onto urban roads could block sewage pipes, causing many inconveniences to people's lives.

In addition, the non-woven masks could pose a severe threat to marine animals once these fragments enter the water reservation, entangling small fishes, crabs, and mollusks (water snails) and causing death by strangulation.⁷ Thus, throwing away the masks or improper waste management of used personal protective equipment can end up causing some problems for wildlife and marine life. As time goes on, these weathered non-woven mask fragments could release several billions of microplastic ﬁbers and particles into the aquatic environment, which will accumulate in the food chain/web.^8,9 If the recycling challenges for non-woven masks are not effectively addressed, used masks will become the next, and large, plastic pollution problem.

Faced with a tremendous number of non-woven waste masks during the COVID-19 pandemic, accelerating the development of a new technology for recycling single-use non-woven masks has become an urgent requirement for protecting our environment and reducing the potential risk of spreading the virus. The historical trends in polymer waste recycling, including non-woven waste masks, are oriented toward centralized facilities transforming the waste into a low-value commodity or energy.^10,11 The recycling techniques of polymer wastes, such as incineration, chemical degradation, and landfill treatment, as well as a high-temperature mechanical blending, are primarily recycling methods for these non-woven waste masks.

However, incineration and chemical treatment, including pyrolysis and gasification, can cause a series of ecological pollution, emitting vast quantities of greenhouse gases and various toxic compounds into the atmosphere, although the available heat can be recovered. The landﬁlling of the waste non-woven masks is also considered the source of secondary environmental pollutants, leaking toxic contaminants into the groundwater and soil.¹²

Recently, high-temperature mechanical blending and recycling have been considered the most environmentally friendly and safest way to upcycle waste masks to produce plastic commodities for further use. Due to high-temperature blending, the bacteria or viruses potentially remaining in the waste non-woven masks can be killed, reducing transmission of the virus or bacterial infection. However, the produced composite materials only convert to a low-value entity without realizing the value from waste non-woven masks, which does not take full advantage of the waste masks and further provide a high economic benefit.

To date, recycled non-woven masks also have a wide variety of applications proposed in the fields of building material,¹³ wastewater treatment,¹⁴ friction nanogenerator devices,¹⁵ oil–water separation membranes,¹⁶ lithium-ion batteries,¹⁷ catalysis,¹⁸ and wearable devices.¹⁹ However, these applications reported in the literature only suggested potential ideas for recycling non-woven masks because many of them remained at the laboratory stage and cannot be quickly put into a practical use when faced with such a large number of used masks during the COVID-19 pandemic every day. Clearly, it is urgent to explore an effective way to address the problem of non-woven waste masks in a safe, economic, and competitive way.

Recently, the emerging fused deposition modeling (FDM) three-dimensional (3D) printing technology demonstrates an attractive opportunity for high value-added application for a wide range of recycled thermoplastic polymer materials.²⁰ It could be an alternative approach to recycle various waste plastic materials by up-cycling small-scale, localized plastic waste. As a matter of fact, various thermoplastic waste materials, such as polylactic acid (PLA),²¹ polyethylene terephthalate (PET),²² polystyrene (PS),²³ polypropylene (PP),²³ and acrylonitrile butadiene styrene (ABS),²⁴ have indeed demonstrated viable feasibility when applied as a 3D printing material. In fact, non-woven masks are mainly made of thermoplastic PP material, which commonly consists of four different parts: the inner layer (skin-friendly non-woven mask fabric), the middle layer (melt-blown ﬁlter), the outer layer (colored non-woven fabric), and the ear band (ﬁber material).²⁵

It is worth mentioning that the PP material from melt-blown ﬁlter and the fabric of the non-woven masks present a relatively small molecular mass range in a narrowed molecular mass distribution. With this unique molecular property, the melt-blown process in the preparation of non-woven masks requires a high melt index and stable rheology from the PP polymer, which could lead to an easier mechanical blending of PP polymer with other additives with sound compatibility.²⁶

For the proposed investigation, non-woven waste masks were recycled and fabricated for a composite material (glass fiber (GF)/recycled polypropylene (rPP)) designed for its potential application as a 3D printing material. A comprehensive evaluation was also carried out to analyze a possible difference between the rPP polymer and a virgin PP material at various GF levels in their chemical/mechanical properties and micromorphological characteristics, as well as the thermal stability of a fabricated filament composite. In addition, comparisons between the composite filament GF/rPP and its 3D printed objects in terms of their mechanical properties was particularly designed to validate our recycled material quality in 3D printing processing.

Clearly, this work aims to evaluate a viable recycling strategy and the associated approach to a large quantity of these disposable non-woven waste masks, which could be possibly applied in 3D printing processing and particularly valued in reducing some negative environmental impacts generated from these non-woven waste masks.

Materials and methods

Non-woven mask collection and their pre-treatment

The used disposable non-woven masks were collected from our campus at Jiangsu University in Zhenjiang, China, in 2021, and some were also collected from those local communities that were identified as low-risk areas of pandemic threat were also used in this research project. The masks were sterilized at 120°C for 20 min by an autoclave device (XFS-280, Xinfeng, China) and then all collections were washed in highly agitated conditions by hand for 5 min with tap water at room temperature, aiming to kill the potential bacteria or viruses remained in the waste non-woven masks and remove residual contaminants. No detergent was used during the washing process. After sterilizing and washing, these mask wastes were dried in a drying oven at 80°C for 12 h in preparation for the hot-press. It should be mentioned that the metal strips and ear bands were removed from the masks after sterilization. The dried mask wastes were layered on top of each other and then hot-pressed into rPP sheets by a heat press machine (ST, Lugong, China) at 160°C and 10 MPa for 20 s, followed by cooling to ambient temperature. The cooled rPP sheets were then subject to being shredded into small ﬂake pieces (<2 cm²) using a manual ﬂat plastic crusher; the overall pre-treatment process starting from collection of the recycling of non-woven mask wastes is demonstrated in Figure 1.

Figure 1.
Pre-treatment process for recycling non-woven mask wastes.

Preparation of GF/rPP composite filaments

To fabricate a qualified candidate material for 3D printing from the rPP materials, two important drawbacks, the warpage and the low accuracy of a 3D printed object, would be the common challenges.²⁷ As our proposed method to address this challenge, the GF (Jiawei Composite Material Co., Ltd, China) was applied to decrease the shrinking percentage of rPP, where the GF additive was added to rPP flakes to fabricate a GF/rPP composite, similar the method reported in the literature.^28–30 The density, tensile strength, and average length characteristics of GF were labeled by the seller as 2.41 g/cm³, 2 GPa, and 159 µm, respectively. Before blending, the GF was treated by a silane coupling agent (Nanjing Chuangshi Chemical Industry Co., Ltd, China), which aimed to form an active functional group on the surface of the GF and a stable bond of –Si–O–Si– among the GF and silane coupling agents, eventually leading to GF-matrix interfacial bonding.^31–34

Next, the uniform mixture of rPP flakes and GF was produced with a twin-screw extruder (SHJ, Nanjing Jieya, China) at a temperature of 175°C (feeding zone), 195°C (molten zone), and 180°C (die zone) to formulate a rPP/GF composite. Then the filaments were successively cooled in a water bath at ambient temperature (about 25°C), pulled into a strand cutter (SHJ, Nanjing Jieya, China), and chopped into pellets. It is noteworthy that the resulting composite filaments (rPP) when blended with the GF element at a lower (<20%) or higher (>40%) percentage by weight would be easily subject to a severe warpage problem or an excessive melt flow challenge during 3D printing with FDM machine.³¹ Thus, the GF element input (wt% of GF/PP) in this investigation was then designed with three different percentage treatments for an optimization test at 20, 30, and 40 wt%, respectively. During the final fabrication step toward a composite filament product, the dried GF/rPP composites pellets were fed into a single-screw extruder (WSJ-12, Xinsuo, China), where there were set up with three processing temperatures at 120°C (feeding zone), 175°C (molten zone), and 160°C (die zone), and then the successive filaments were cooled in a water bath (WSC-500, Xinsuo, China) at ambient temperature (about 25°C). The extrusion speed and traction speed were simultaneously maintained at 100 r/min and 450 mm/min, respectively.

As a matter of fact, the traction speed of the puller we selected was first subject to an optimization trial before the ﬁlaments were successfully fabricated. Once the filament formulation was stabilized at 1.75 ± 0.05 mm diameter, the composite filament products were eventually applied for the following FDM 3D printing evaluation. As a schematic display, the overall steps involved in the GF/rPP composite filament preparation as mentioned above are briefly shown in Figure 2.

Figure 2.
Preparation process of recycled polypropylene (rPP) composite filaments. GF: glass fiber.

For the quality evaluation of the GF/rPP composite material recycled from non-woven waste masks, the purchased PP products (virgin PP, Taiwan Yongjia Chemical Industry Co., Ltd, China) were also subject to the same preparation process as mentioned above for fabrication of a GF/PP composite to evaluate a potential difference between the virgin GF/PP material (which served as a control) and the GF/rPP composites.

As for the composite product comparison, the GF volume fraction ( $Vol %$ ) and the theoretical density ( $ρ_{composite}$ ) of the composite filaments at different GF percentage inputs with PP and rPP matrices are given by Equations (1)³² and (2),³² respectively
$Vol % = \frac{w \cdot ρ_{polymer matrices}}{w \cdot ρ_{polymer matrices} + (1 - w) \cdot ρ_{G F}}$
(1)

$ρ_{composite} = \frac{ρ_{G F} \cdot ρ_{polymer matrices}}{w \cdot ρ_{polymer matrices} + (1 - w) \cdot ρ_{G F}}$
(2)
where $Vol %$ is defined as the GF volume fraction and $w$ is denoted as the fiber weight. Here, $ρ_{G F}$ , $ρ_{polymer matrices}$ , and $ρ_{composite}$ are the density of GF, polymer matrices, and composite filaments, respectively. The applied densities of PP and rPP measured by a density measuring instrument (BOS-100 A, Boshi, China) in our calculation were 0.858 and 0.9 g/cm³, respectively. The resulting GF volume fraction and theoretical density of the composite filaments are presented in Table 1.

Table 1.
The glass fiber (GF) volume fraction and theoretical density of composite filaments at different composite filament compositions

Composite filament composition GF volume fraction (%) Theoretical density (g/cm³)

20 wt% GF/rPP^a 8.162 1.029

30 wt% GF/rPP^a 13.222 1.109

40 wt% GF/rPP^a 19.159 1.201

20 wt% GF/PP^b 8.526 0.985

30 wt% GF/PP^b 13.777 1.064

40 wt% GF/PP^b 19.907 1.156

^aComposite filament from waste masks.

^bComposite filament from virgin PP.

rPP: recycled polypropylene; PP: polypropylene.

In addition, as pores/voids are generated during the filament fabrication processing, as well as 3D printing processing, the density of each 3D printing filament and its 3D printed object from GF/PP and GF/rPP composites at a GF level was subject to measurement by a density measuring instrument (BOS-100A, Boshi, China). In addition, the porosity values were further calculated in terms of measured density by the following equation
$η = \frac{ρ_{s -} ρ_{composite}}{ρ_{s}} \times 100 %$
(3)
where $η$ is defined as the porosity, ρ_s is denoted as a theoretical density of a composite filament at a GF level, and ρ_composite is the measured density value from a 3D printing filament or a 3D printed object from GF/PP or GF/rPP composite at a GF level.

Chemical and mechanical properties

An X-ray diffractometer (XRD, D8, ADVANCE, Germany) was used to analyze the crystal structure of virgin PP, rPP from waste masks, and the composite materials of GF/PP and GF/rPP. The composite samples were tested after being heated from an ambient temperature of about 25°C to 200°C and cooled to room temperature. The instrument scanning angle during the test was set up from 5° to 50° with a scanning rate of 5°/min. In addition, all test materials mentioned above were also analyzed for their chemical composition identified with their Fourier transform infrared (FTIR) spectrum (Bruker FTIR spectrometer, ALPHA II, Bruker, Germany).

As for the mechanical property regarding composite materials, the filaments of GF/PP and GF/rPP were also evaluated by a universal mechanical testing machine (E43, MTS, America) with a filament testing ﬁxture (Fundamental). During the testing, the filaments were wound in fixtures with an actual test length of 60 mm and the loading velocity was set up at 2 mm/min under a force of 500 N transducer capacity.

Thermal properties

The virgin PP, rPP from waste masks, and the composite filaments of GF/PP and GF/rPP were evaluated for their thermal transition performances by a differential scanning calorimeter (DSC; DSC4000, PerkinElmer, USA). Each dried test sample, about 8 mg, was designed to undergo two thermal cycles, heating from 30°C to 180°C and then cooling to 30°C at a rate of 10°C/min in a nitrogen atmosphere, where the second thermal cycle data was eventually collected. The main thermal parameters, including the melting temperature ( $T_{m}$ ), the crystallization temperature ( $T_{c}$ ), and the percentage of crystallization ( $X_{c}$ ), were reported from the DSC curves. The crystallinity index, $X_{c}$ , was calculated in the following equation
$X_{c} = \frac{△ H_{m} - △ H_{c c}}{△ H_{f}} \times \frac{100}{W_{P P}}$
(4)

Here, ${△ H}_{m}$ is the melting enthalpy of those samples, $△ H_{f}$ is the PP 100% crystallization enthalpy, $W_{P P}$ is the weight fraction PP or rPP net content, and $△ H_{f}$ is approximately denoted at 210 J/g.

In addition, the thermal stability for each material sample was also subject to measurement by thermal gravimetric analysis (TGA, TGA4000, PerkinElmer, USA), using the temperature range of 30–600°C at a heating rate of 10°C/min in a nitrogen atmosphere.

As an important property of each fabricated composite material, the rheological performance for each sample was further evaluated by a rheometer (MARS III, Thermo Scientific Haake), where the viscosity data were measured during the testing at a fixed temperature of 200°C, and a strain amplitude of 1% with shear frequencies from 0.1 to 500 s⁻¹.

FDM 3D printer parameter settings

In our design, a sample of 3D printing was fabricated by a FDM desktop 3D printer (Z603s, JGAURORA, China) for each prepared filament material, including the composites of GF/PP, GF/rPP, and a commercial PP product (where served as the control), where Cura-Slicing software was employed to process the computer-aided design (CAD) models for each material we tested. The primary 3D printing parameters are listed in Table 2. Lastly, a dumbbell-shaped, columnar-shaped, strip-shaped test object, as well as the density test sample, were fabricated by the FDM desk 3D printer as mentioned above in terms of the printing parameters presented in Table 2.

Table 2.
Fused deposition modeling three-dimensional printing parameters applied for glass fiber (GF)/recycled polypropylene and GF/polypropylene (PP) composite filaments

Printing parameters Value

Nozzle diameter (mm) 0.5

Nozzle temperature (°C) 200

Platform temperature (°C) 70

Layer thickness (mm) 0.2

Printing speed (mm/s) 50

Infill density (%) 100

Wall thickness (mm) 1

Infill pattern Lines

Raster angle (°) [–45, +45]

Overlap interval (mm) 0

Build platform type PP plate

Build platform thicknesses (mm) 5

The 3D printed test objects of different shapes and dimensions are exhibited in Figure 3, which will be applied to the designed comprehensive mechanical and morphological property evaluations.

Figure 3.
Fused deposition modeling three-dimensional (3D) printed samples, including a physical picture (a) and a 3D view (b). GF: glass fiber; rPP: recycled polypropylene; PP: polypropylene.

Mechanical and morphological properties of the printed objects

A comprehensive mechanical property evaluation for each of the 3D printed objects with composite filaments of GF/PP and GF/rPP, as well as the virgin PP, were tested by a mechanical testing machine (E43, MTS, USA) under a 500 N transducer capacity and an impact tester (LX-XJJ-118, Huisite, China), where the impact tester was equipped with a 5.5 J pendulum according to the standards ISO 527-2: 2012 (tensile test), ISO 178: 2010 (flexural test), ISO 604-2002 (compression test), and ISO 179-1: 2010 (impact test). The loading velocity of the tensile test, flexural test, and compression test were measured with the parameters being set up at 2, 1.3, and 1 mm/min, respectively.

The microstructure of fractured tensile object surfaces was observed by a scanning electron microscope (S-3400, Hitachi, Japan) to analyze their internal defects as well as the potential fracture mechanism. In addition, the surface topography and roughness of the FDM 3D printed objects were also evaluated according to the standard ISO 25178 by laser scanning microscopy (OLS5000-SAF, Olympus, Japan). The arithmetical mean height (Sa) was further recorded at three different positions of the horizontal forming surface, where an average value was then calculated.

Furthermore, the geometrical accuracy of a 3D printed object was tested by the cube model and quantified by dimensionless speciﬁc warpage (Wsp) as shown in Figure 4.

Figure 4.
Fused deposition modeling three-dimensional printed cube sample at 20 wt% glass fiber (GF)/recycled polypropylene (rPP) (a) and its schematic calculation of geometrical accuracy (b). PP: polypropylene.

The W_sp index was then calculated using the following equation
$W_{s p} = \frac{W_{abs}}{h_{\max}}$
(5)
where $W_{abs}$ is defined as the maximum height of the edge from the plane of the print substrate and $h_{\max}$ is the maximum height of the testing objects.

During the geometrical accuracy testing, a cube model with sides of 20 mm and infill density of 20% was 3D printed for $W_{abs}$ and $h_{\max}$ measurement. In addition, the geometrical accuracy of a 3D printed PP sample was also significantly impacted by the selection of a building platform material during the 3D printing, so two kinds of platforms, a virgin PP plate (3 mm thick, Zhenhongbo Co., Ltd, China) and PP tape (No. 830, Sekisui Chemical Co., Ltd, Japan), were further used to analyze a potential differential of geometrical accuracy for 3D printed objects from both GF/PP and GF/rPP composites.

Data analysis

Sample preparations of each fabricated composition material and its property evaluation tests in terms of the mechanical properties, geometrical accuracy, and surface roughness, as well as density testing, were repeated in triplicate for statistical analysis. Error bars in all figures represent the difference in triplicate tests. The mean comparisons in geometrical accuracy, surface roughness, and mechanical properties among three composition treatments of GF/rPP composite samples at 20, 30, and 40 wt% GF, as well as the GF/PP composite (which served as a control group at the three 20, 30, and 40 wt% GF levels) and the virgin PP (which served as a control group only in mechanical property comparisons), were conducted by one-way analysis of variance (ANOVA) testing and Tukey multiple comparison, respectively. To confirm a potential quality difference in material between 3D printed objects and their corresponding filaments prior to printing, the mean comparisons in density of each 3D printed GF/PP and GF/rPP composite object and filament were also analyzed by one-way ANOVA and Tukey multiple comparison. The statistical data analyses for mean comparisons as mentioned above were proceeded at the P-value < 0.05 and run by SPSS statistical software (SPSS 23, IBM Corp., USA).

Results

Chemical properties

The crystallinity and crystal types of PP and rPP, as well as the composites of GF/PP and GF/rPP, pre-treated by a heat–cool cycle (hot-pressed at 160°C and 10 MPa for 20 s followed by cooling to ambient temperature) were analyzed by an XRD (Figure 5(a)). All materials were indicated as five reflections at 14.1°, 16.9°, 18.5°, 21.1°, and 25° associated with the (110), (040), (130), (111), (131), and (041) reflections, respectively, which would be very likely related to a structure of the monoclinic PP α-crystal.³⁵ This suggested that the addition of GF and the recycling process caused no distinct modification to the crystal structure of the PP matrix. Besides, the crystal intensity of GF/PP composites was lower than that of GF/rPP composites when the GF content was added at the same level. However, an extra peak at 16.1° and an intensified peak at 21.1° (matching the (300) and (301) planes of PP hexagonal β-crystals) were uniquely observed in the 3D printed objects (Figure 5(c)).

Figure 5.
The X-ray diffractometer (XRD) (a) and Fourier transform infrared spectra (b) were recorded from glass fiber (GF), polypropylene (PP), and recycled polypropylene (rPP), as well as the GF/PP and GF/rPP, filaments at various GF levels (20, 30, and 40 wt%), prepared by a controlled heat–cool cycle; the XRD spectra were observed from GF/PP and GF/rPP composites at the 20 wt% GF level prepared by a controlled heat–cool cycle or produced from a 3D printing object (c).

During the FDM 3D printing process, the induced orientations, high shear rates, and annealing at the higher temperature resulted in a mixture of PP α- and β-crystals in each 3D printed object of GF/PP and GF/rPP composites. Furthermore, the relative intensities of reflections between 3D printed objects and the heat–cool cycle treated samples were different, especially at the (110), (040), and (130) planes (Figure 5(c)). The manufacturing processing might be responsible for the disparity in the overall intensities. The GF was crushed during the heat–cool cycle process by a hot-press machine, as well as a quick cooling of melting materials, which might contribute to a high crystallinity in a heat–cool cycle treated sample.

The FTIR spectra of PP and rPP, as well as the GF/PP and GF/rPP composite filaments, at various GF contents are shown in Figure 5(b). The peaks located at around 1459 and 1376 cm⁻¹ were typical of PP, corresponding to －CH₂ and－CH₃, respectively, indicating that the main component recovered from waste masks was indeed a PP material. Due to the melt-blending process between the GF and rPP, a slight reduction in absorption peaks was found for the GF/rPP composite samples in nearly all characteristic peaks if compared with the rPP matrix.

Material thermal property and its rheology behaviors

The main thermal parameters, melting temperature (T_m) and crystallization temperature (T_c), for virgin PP, rPP from waste masks, and the composite filaments of GF/PP and GF/rPP were all evaluated in terms of a DSC analysis. The T_m and T_c value of GF/rPP composite filaments were almost the same as those of rPP, as shown in Figures 6(a) and (b), indicating that the addition of GF has no an observed effect on the crystal sphere size and its isotacticity toward the rPP matrix. However, the T_m and T_c of GF/PP composite filaments showed an increasing and decreasing trend, respectively, when compared to PP curves, which demonstrated that the addition of GF would potentially strengthen the thermal stability of the PP matrix as an increase at the GF weight fraction.

Figure 6.
The differential scanning calorimeter curves at heating scan and their melting temperature, T_m (a) and curves at the cooling scan and their crystallization temperature, T_c (b), thermal gravimetric analysis curves (c), and derivative thermogravimetry (DTG) curves (d) for polypropylene (PP) and recycled polypropylene (rPP), and glass fiber (GF)/PP and GF/rPP composite filaments.

The thermal stability characteristics for PP and rPP, as well as the GF/PP and GF/rPP composite filaments, were further evaluated using derivative thermogravimetry (DTG) and TGA, where the weight loss temperature at 5% composite mass loss (T_5%) was applied as an initial decomposition temperature. Clearly, with this investigation (Figures 6(c) and (d)), it can be seen that T_5% of both GF/rPP and GF/PP composite filaments was significantly improved along with a higher GF weight fraction and was better than that of the PP and rPP matrix in the cases of 55.17°C and 21.92°C at the 40 wt% GF treatment. These observations indicated that the GF material might play an important role in preventing the destruction of PP chains or a dehydrogenation action during the processing in the thermal decomposition stage.

As a matter of fact, these results for the function of GF material in a composite were largely attributed to the active functional groups existing on the GF surface that were modified by silane coupling agents for a stronger interfacial bonding performance. The weight percent loss data recorded for all composites after complete degradation of the polymer are demonstrated in Figure 6(c), which indicated that the melt-blending processing between the GF and the polymer matrix was a homogeneous performance. On the other hand, the DTG curves (Figure 6(d)) showed that the temperature at the maximum weight loss rate (T_v_max) for a GF/PP composite was enhanced when the GF material was introduced at different levels. However, no temperature enhancement in the maximum weight loss rate was observed for any of the GF/rPP composites, which may be possibly due to a weaker bonding effect from the rPP matrix than that of PP performed at a high temperature.

The evaluations of the main thermal parameters, T_m, T_c, degradation temperature (T_d), T_v_max, and percentage of crystallization (X_c), for PP and rPP, as well as the GF/PP and GF/rPP composite filaments, at various GF contents are presented in Table 3. The X_c for each tested material was calculated by Equation (4). An inferior crystallinity performance was recorded in GF/rPP and GF/PP composite filaments along with an increase in GF weight fraction, which indicated that the movement of the polymer molecular chain during the crystallization process was limited by immense interaction between the GF and the polymer matrix.

Table 3.
Thermal property evaluations demonstrated with various parameters of the recycled polypropylene (rPP) and polypropylene (PP) matrices, as well as the glass fiber (GF)/PP and GF/rPP composite filaments

Filament composition T_m (°C) ΔH_m (J/g) T_c (°C) X_c (%) T_d (°C) T_v_max (°C) Weight loss (%)

rPP^a 164.46 121.17 121.82 57.98 346.51 438.58 99.482

20 wt%GF/rPP^a 163.72 98.15 121.65 58.7 358.52 435.36 80.996

30 wt%GF/rPP^a 163.63 79.64 121.86 54.47 365.37 437.23 69.782

40 wt%GF/rPP^a 163.93 67.01 121.71 53.43 368.43 436.9 61.29

PP^b 150.68 87.84 119.87 42.02 319.59 417 99.878

20 wt%GF/PP^b 152.04 61.02 119.53 36.5 358.52 446 80.999

30 wt%GF/PP^b 151.05 53.91 118.85 36.87 365.88 441.6 72.236

40 wt%GF/PP^b 151.15 45.35 118.13 36.1 374.76 440.6 64.138

^aComposite filament from waste masks.

^bComposite filament from virgin PP.

The rheology behavior for a type of 3D printed material is critical in determining some key parameter settings with a FDM 3D machine, especially for the nozzle temperature set up and suitable printing speed selection. Therefore, the rheology curves of PP and rPP, as well as the GF/PP and GF/rPP composites, were depicted with shear frequencies from 0.1 to 500 s⁻¹. A typical homopolymer curve could be observed in each material (Figure 7), with a Newtonian plateau in a low shear rate range, and shear-thinning behavior evolved as the frequency increased. The melt flowing property of the rPP matrix was obviously higher than that of the virgin PP matrix. Besides, each composite of GF/PP and GF/rPP exhibited a higher viscosity value than those of the PP and rPP matrices. The complex GF structure was organized in the rPP and PP matrices when a higher GF weight fraction was added, which strongly hindered the flowability of the GF/PP and GF/rPP composites in a low shear frequency. However, enhanced shear-thinning behavior was recorded in the GF/PP and GF/rPP composites at a high GF level when the shear rate was increased.

Figure 7.
Rheology property observed for glass fiber (GF)/recycled polypropylene (rPP) (a) and GF/polypropylene (PP) (b) composites at varied GF weight fractions.

Mechanical property for filament composites

The tensile strength and elongation property at break for each fabricated composite filament of GF/rPP and GF/PP, as well as a commercial PP filament product, were evaluated and are presented in Figures 8(a) and (b).

Figure 8.
Tensile strength (a) and elongation at break (b) for glass fiber (GF)/polypropylene (PP), GF/recycled polypropylene (rPP) composite filaments, the commercial PP filament, and rPP filaments. (The same letters that are denoted in each column of GF/rPP, GF/PP, and commercial PP show an insignificant difference (P < 0.05).)

It is noteworthy that a significant decreasing trend was presented in the tensile strength of GF/rPP composite filaments along with a higher GF weight fraction from 20 wt% GF (27.78 ± 0.885 MPa) to 40 wt% GF (19.67 ± 1.183 MPa) (Figure 8(a)). In addition, the tensile strength of GF/rPP composite filaments was significantly higher than that of GF/PP composite filaments at 20 and 30 wt% GF, respectively, which may be possibly due to a difference in their crystallinity and rheological property. A higher melt fluidity and higher crystallinity of GF/rPP composite filaments would commonly present a higher tensile strength than that of GF/rPP composite filaments.

On the other hand, the elongation performance at break for each filament of the GF/rPP and GF/PP composites, as well as the virgin PP product, was also tested (Figure 8(b)). Clearly, the elongation data at break demonstrated an insignificant difference among each treatment at 20, 30, and 40 wt% GF for GF/rPP composite samples. However, the elongation values at break for GF/rPP filaments was significantly lower than that of both the GF/PP filaments at the same GF level and the virgin PP filament.

Mechanical property for 3D printed objects

A comprehensive mechanical property evaluation for a 3D printed object from various GF/PP and GF/rPP composite filaments, as well as a commercial PP filament product, was designed and conducted (Figure 9), which mainly included five different mechanical properties, that is, tensile strength (a), elongation at break (b), compression strength (c), flexural strength (d), and impact strength (e).

Figure 9.
Five essential mechanical properties and their comparisons at different glass fiber (GF) levels, namely tensile strength (a), elongation at break (b), impact strength (c), compression strength (d), and flexural strength (e) for fused deposition modeling three-dimensional printed objects from GF/polypropylene (PP) and GF/recycled polypropylene (rPP) composites and commercial PP filaments, as well as the stress–strain curves (f) for 20 wt% GF/PP and 20 wt% GF/rPP in the tensile strength test. (The same letters for data noted in each column of GF/rPP, GF/PP, and commercial PP show an insignificant difference at P < 0.05.)

When a comparison was set up at a fixed GF level, a certain type of mechanical property between 3D printed objects fabricated from GF/rPP and GF/PP composites may present some differences. The elongation value at break and impact strength of a 3D printed object from the GF/rPP composite was significantly lower than that of GF/PP objects at each GF level, respectively (Figures 9(b) and (e)). As for another important mechanical property, the flexural strength of a 3D printed object from the GF/rPP composite was also significantly lower than that of GF/PP objects at the 20 and 30 wt% GF treatment (Figure 9(d)).

Interestingly, the compression strength for a 3D printed object from the GF/rPP composite was significantly higher than those of GF/PP samples at 20 and 30 wt% GF (Figure 9(c)). For the tensile strength property, the strength performance of a 3D printed object from GF/rPP was significantly higher than those of GF/PP objects at 20 and 40 wt% GF (Figure 9(a)). In addition, the tensile fracture performance for a 3D printed object was further evaluated (Figure 9(f)), where a 3D printed object from the GF/rPP composite was relatively brittle to fracture if compared with those of GF/PP composite objects, which commonly presented a ductile necking characteristic.

In addition, to reveal a potential difference in mechanical properties between 3D printed objects fabricated from GF/rPP composite filaments and the commercial PP product, further comparisons were designed and conducted. As for the compression, that is, the tensile and flexural property, its strength performance of a 3D printed object with GF/rPP was significantly higher at 20 wt% GF than those of commercial PP objects (Figure 9(c)). As a matter of fact, with respect to the impact property, no significant difference in the impact strength for a 3D printed object from GF/rPP composites was detected at 20 wt% GF levels if compared with that of the commercial PP product (Figure 9(e)).

The mechanical properties of a 3D printed object from GF/rPP composites showed a significant decrease in tensile strength, compression strength, flexural strength, and impact strength when a higher GF weight fraction was applied. However, no significant difference in the elongation property at break for a 3D printed object was observed from GF/rPP composites at three different GF levels (each test result is presented in Table 4).

Table 4.
Mechanical properties observed for fused deposition modeling three-dimensional printed samples fabricated from glass fiber (GF)/recycled polypropylene (rPP) and GF/polypropylene (PP) composites and a commercial PP product

Mechanical properties at varying GF contents GF/rPP* GF/PP* Commercial PP product*

Tensile strength (MPa)

20 wt% 19.06 ± 1.12^a 17.74 ± 0.12^b 17.17 ± 0.93^b,c

30 wt% 17.25 ± 0.41^bc 16.27 ± 0.16^bc

40 wt% 16.23 ± 0.32^c 14.83 ± 0.57^d

Elongation at break (%)

20 wt% 4.41 ± 0.19^c 23.18 ± 4.00^a 15.21 ± 2.66^b

30 wt% 3.79 ± 0.15^c 19.19 ± 3.04^a,b

40 wt% 3.32 ± 0.57^c 16.10 ± 0.85^b

Impact strength (MPa)

20 wt% 9.88 ± 1.27^b,c 15.78 ± 1.68^a 10.33 ± 1.05^b

30 wt% 7.31 ± 0.95^b,c 14.44 ± 1.43^a

40 wt% 4.19 ± 0.71^d 11.04 ± 1.60^b

Flexural strength (MPa)

20 wt% 26.51 ± 0.70^b 32.69 ± 1.47^a 17.94 ± 0.65^d

30 wt% 22.74 ± 0.48^c 30.8 ± 0.73^a

40 wt% 22.26 ± 1.66^c 20.89 ± 1.11^c

Compression strength (MPa)

20 wt% 32.85 ± 2.84^a 17.08 ± 0.78^c,d 19.86 ± 0.78^b,c

30 wt% 22.09 ± 3.87^b 15.44 ± 0.44^c

40 wt% 16.09 ± 1.21^c,d 14.92 ± 0.78^c

The same letters in each evaluated property section (GF/rPP, GF/PP, and commercial PP) indicate an insignificant difference at P < 0.05.

Microstructure observations of the composite material

In our design, scanning electron micrographs of each cross-section from GF/PP and GF/rPP composite filaments at 20 and 40 wt% GF levels were analyzed for their microstructure characteristics (Figure 10). Our micrographs indicated that a brittle tensile fracture was commonly observed from the cross-section of those fabricated GF/rPP composite filaments (Figures 10(a) and (b)), but was not presented as a ductile tensile fracture that was often presented in GF/PP composite filaments (Figures 10(c) and (d)). Moreover, it was found that the shape of the filament in cross-sections was close to a circle in each tested sample (Figures 10(a1)–(d1), but a slight shrinking was observed for 20 wt% GF/rPP filaments (Figure 10(a1)), possibly due to their higher crystallinity. The relatively evenly distributed GF material inputs and some voids were clearly observed and scattered on those cross-sections from GF/rPP composite filaments. Nevertheless, a lower densification with more voids in a cross-section from the 40 wt% GF/rPP filament can be noticed, when compared with that of a surface of a 20 wt% GF/rPP filament (Figures 10(a) and (b)). Besides, the inconsistent shrinkage force between polymer materials and the added GF material during the cooling processing would potentially result in pore enlargement on their interfaces, which can be easily evidenced from the magnified micrograph in Figure 10(a2). However, in most cases, it can be easily found that the GF element was typically wrapped by the polymer material, which demonstrated a sound interfacial adhesion, as shown in Figure 10(b2).

Figure 10.
Scanning electron micrographs of the brittle tensile fracture surfaces for the 20 wt% glass fiber (GF)/recycled polypropylene (rPP) (a1), (a2) and 40 wt% GF/rPP (b1), (b2) filaments, and the ductile tensile fracture surfaces at 20 wt% GF/polypropylene (PP) (c1), (c2) and 40 wt% GF/PP (d1), (d2) filaments.

Microstructure observations of the 3D printed objects

To reveal a potential mechanism at break for a 3D printed object fabricated from GF/rPP or GF/PP composite material, the fracture surfaces in magnified detail recorded by scanning electron microscopy (SEM) from a tensile test at break were further analyzed (Figure 11). In a 3D printed object from the GF/rPP composite, it was found that a brittle fracture surface was clearly denoted on the fracture sections at any GF percentage treatment (Figures 11(a1), (b1), and (c1)). A growing number of voids was easily observed along with a higher GF percentage treatment in Figures 11(a1)–(c1).

Figure 11.
Scanning electron micrographs of tensile fracture surfaces for three-dimensional (3D) printed samples from glass fiber (GF)/polypropylene (PP) and GF/recycled polypropylene (rPP) composites at 20% (first column), 30% (second column), and 40% GF weight fraction (third column), where the small air pores marked with yellow dashed circles were mainly produced during the fabricating processing of the filaments, the small pores marked with red dashed circles were generated at a break during the tensile test, the oval air pores marked with blue dashed circles were primarily formed during the 3D printing processing, and the interlayer boundaries marked with red dashed lines were generated due to the 3D printing processing. (Color online only.)

In these voids, the small pores were very likely generated during the inhomogeneous mixture processing between the GF material and the polymer matrix, as well as the following extrusion processing for filaments, as shown in Figure 11(c1), where they are marked with yellow dashed circles. However, as a matter of fact, some small pores, marked with the red dashed circles in Figure 11(a1), were identified as those pores generated from a break. Besides, oval air pores, as marked by blue dashed circles in Figures 11(b1) and (c1) and presented as the interlayer gaps between stacked paths and layers, were primarily formed during the 3D printing processing. In addition, the observable voids around the cross-section boundary in a 3D printed object from GF/rPP composites might potentially lead to a quick crack from the void concentrated places during a mechanical property test upon a break force being applied.

Interestingly, it has also been found that the GF was tightly wrapped by the rPP polymer in its fracture surfaces after a tensile test for most of the 3D printed objects from GF/rPP composites, as shown in Figures 11(a2), (b2), and (c2). However, when the 40 wt% GF was applied to fabricate the GF/rPP composite, a GF aggregation phenomenon occasionally occurred in the fracture surfaces after the tensile test applied to a 3D printed object, which may cause a quicker crack propagation.

The microstructure characteristics of a fracture surface have demonstrated a difference among the 3D printed samples after a tensile test, mainly due to a variation in their composite contents, such as GF/rPP or GF/PP. As a matter of fact, a ductile fracture surface was often presented after a tensile test of a 3D printed object from the GF/PP composite, which was quite different from the brittle fracture surface that commonly occurred for a 3D printed object from GF/rPP at any GF level (Figures 11(d1), (e1), and (f1). In addition, an observable change in the ductile length of the PP matrix was recorded on a fracture surface along with a change in GF percentage levels for a 3D printed object from GF/PP composites.

Furthermore, it has also been found that the GF additive wrapped by the PP matrix in the GF/PP composites was partially separated from the PP matrix after a tensile test at break, which would become a severe separation when a higher GF percentage was applied to the GF/PP composite (Figures 11(d2), (e2), and (f2)). This phenomenon further demonstrated weaker interfacial infiltration and adhesion between the GF additive and the polymer matrix in GF/PP composites than GF/rPP composites.

Geometrical accuracy evaluations

In general, the extraordinary susceptibility in shrink and warpage of the PP is a primary disadvantage in utilizing rPP material during FDM 3D printing processing. In order to identify this potential disadvantage for our 3D printed objects, PP tape and PP plate were selected as the 3D printing build platform to ensure and evaluate a sound repeatability and printability for a 3D printed object from GF/rPP composites.

The specific warpage indexed by Wsp* for each 3D printed cubic object from GF/rPP and GF/PP composites was calculated by Equation (5). It has been found that the specific warpage parameter, Wsp, for a 3D printed object built with PP plate from GF/rPP and GF/PP composites was significantly lower than that of a 3D printed object built with PP tape (Figure 12). This phenomenon indicated that PP plate might possess a stronger adhesion force than that of PP tape between the ﬁrst layer of a 3D printed object and the build platform, which would very likely counteract the force induced by warpage and residual stress in each 3D printed object from GF/rPP and GF/PP composites. In addition, no significant difference in the Wsp index for a 3D printed object built with PP plate from GF/rPP and GF/PP composites was observed among various GF percentage treatments.

Figure 12.
The specific warpage values were compared for three-dimensional printed cubic object builds in the formats of polypropylene (PP) tape and PP plate from glass fiber (GF)/recycled polypropylene (rPP) and GF/PP composites at varied GF weight fractions, where the same letters in each column show an insignificant difference at P < 0.05.

Based on a future practical application, the 3D printed object from GF/rPP composites should not only present some accepted comprehensive mechanical properties, but also needs to meet some minimum requirements for its surface quality. In this regard, surface roughness (Sa) and surface topography for 3D printed objects from GF/rPP and GF/PP composites at different GF treatments were subject to evaluation by a laser scanning microscope. Interestingly, many deposited paths could be clearly observed on the surface of the 3D printed object from GF/rPP composites (Figures 13(a1)–(a3)). However, on the surface of the 3D printed objects from GF/PP composites, irregular stacking on the deposited paths was often observed (Figures 13(b1)–(b3)). The deposited paths in a different way might possibly be formed by higher viscosity in the nozzle channel to GF/PP composites than that to GF/rPP composites at a GF level during the 3D printing process.

Figure 13.
Surface topography and surface roughness (Sa) were imaged and compared for fused deposition modeling three-dimensional printed objects from glass fiber (GF)/polypropylene (PP) and GF/recycled polypropylene (rPP) composites at varied GF levels.

In addition, the surface roughness indexed by Sa for the 3D printed object from GF/rPP composites showed a significant decreasing trend when a higher GF weight fraction was applied. Surprisingly, the Sa index value was significantly higher for 3D printed objects from GF/rPP composites than that of 3D printed objects from GF/PP composites at the same GF level (P = 0.002 for 20 wt% GF , P < 0.001 for 30 wt% GF, P = 0.007 for 40 wt% GF), which could be due to a diameter variation in their GF/rPP filaments. As a matter of fact, the surface quality of 3D printed objects from GF/rPP composites could be further improved and optimized by adjusting their 3D printing parameters.

Comparison of key properties between composite filaments and their 3D printed objects

To investigate a potential difference in mechanical properties between the fabricated composite filaments and their 3D printed objects from the GF/PP and GF/rPP composites, comparisons of material density (g/cm³), including its theoretical value calculated by Equation (3), were conducted. In the data to be presented in Figure 14(a), each density value measured by a density device for the 3D printing filaments and their 3D printed objects was significantly lower than its theoretical density at each GF level, suggesting that some voids/pores were produced during the filament fabrication, as well as the following 3D printing processing.

Figure 14.
The theoretical density and recorded densities of three-dimensional (3D) printing filaments and their 3D printed samples from glass fiber (GF)/polypropylene (PP) and GF/recycled polypropylene (rPP) composites compared at varied GF levels, where the same letters each column show an insignificant difference at P < 0.05 (a). The porosity values were further calculated and compared for 3D printing filaments and 3D printed samples from the GF/PP and GF/rPP composites at varied GF weight fractions (b).

A higher porosity value from each 3D printed object, shown in Figure 14(b), was clearly observed than that of the filament material at each GF level, which may very likely result in an easier crack consequence for a 3D printed object during the mechanical property test when compared with its filament composite. In fact, a possible material effect from the polymer matrix we used may play a role in its porosity value during the material processing, where porosity values at 5% and 6% lower for rPP/GF composites than those of GF/PP composite were indeed recorded during the filament fabrication and the 3D printing processing, respectively.

Our further observations demonstrated that the porosity values altered during the filament fabrication processing were estimated in the ranges of 0.8–1.6% and 6–7.2% for the fabricated filaments from GF/rPP and GF/PP composites, respectively. Besides, the following porosity values to be enhanced during the 3D printing processing were further estimated in the ranges of 7.4–9.2% and 13.4–15.4% in the 3D printed objects from the GF/rPP and GF/PP composites, respectively.

Interestingly, a higher porosity value was constantly retained with those composites fabricated from the purchased PP matrix when compared with the composites fabricated from the rPP matrix, regardless of the GF percentage level applied (Figure 14(b)), suggesting that the rPP matrix with low viscosity may possibly contribute to more densification than the PP matrix during the composite fabrication.

Discussion

The proposed sterilization and hot-press pre-treatment steps are a very important processing design that is suitably applied in recycling PP from non-woven waste masks. The potential bacteria or viruses remaining in the waste non-woven masks could be completely killed during the sterilization and hot pre-treatment processing. In addition, these collected non-woven waste masks were highly compacted during the hot-press processing, which allowed the advantages of easier manipulation and enhanced property performance in the following composite development and filament fabrication.

The application of GF material to develop a GF/rPP composite is an essential procedure to improve its filament fabrication that will be applied in the next step for 3D printing processing. With this unique design, our investigations have brought us three significant advantages, summarized as the follows: (1) the thermal stability of GF/rPP composites was significantly enhanced when compared with the rPP matrix, where the GF material may possibly play an important role in preventing the destruction of PP chains or a dehydrogenation action; (2) the GF material would indeed serve as a tackifier to improve the melt viscosity of GF/rPP composites during the filament fabricating processing as well as its following 3D printing; (3) impressively, an observable inferior crystallinity of the GF/rPP composites was recorded primarily due to the added GF material, which may play a role in improving its geometric accuracy for 3D printed objects.

The GF additive at 30 wt% is an optimized and recommended proportion to mix with the rPP matrix in fabricating the GF/rPP composite filament for 3D printing processing. Based on our evaluations, the filaments fabricated by GF/rPP composites at 30 wt% GF material not only ensure a sound repeatability and printability for 3D printed objects, but also achieve an acceptable mechanical property performance for a 3D printed object. In addition, the surface roughness (Sa) of a 3D printed object from the GF/rPP composite at 30%wt could reach its printing accuracy at 21.1 ± 0.76 µm and no clogging phenomenon was observed in the printer nozzle channel during the 3D printing processing, suggesting that the compatibility of this fabricated composite filament is well matched with our desktop FDM 3D printing machine.

The porosity recorded on the surface of the fabricated GF/rPP composite material as well as its 3D printed object may possibly serve as a crucial indicator in evaluating its mechanical property performance in a 3D printed object. The microstructure observation of a 3D printed object from GF/rPP composites material showed that some small pores recorded on the surface were very likely generated during the inhomogeneous mixture processing between the GF additive and the rPP matrix, as well as the following extrusion processing for filament fabrication. As a matter of fact, some oval air pores were also observed during the 3D printing processing, where the porous structure around the cross-section boundary in a 3D printed object from GF/rPP composites might potentially lead to a quick crack from the void concentrated places during a mechanical property test upon a breaking force being applied.

As a matter of fact, some observable differences in mechanical properties in the 3D printed objects existed between GF/rPP and GF/PP composite materials. The flexural strength and impact strength of a 3D printed object from the GF/PP composite are significantly higher than those of 3D printed objects from GF/rPP composites, which may be due to a difference in the molecular weight of their polymer matrices. A shorter molecular chain of the rPP polymer matrix in a 3D printed object from GF/rPP composites would commonly present an inferior transferring and dispersing capability than that of the GF/PP matrix when an external flexural and impact force is applied during the mechanical test.^36,37 However, the other two properties, compression strength and tensile strength, for a 3D printed object from the GF/rPP composite were significantly higher than that of a 3D printed object from the GF/PP composites at any GF level, which may be largely due to the differences in their rheology and crystallinity properties. In fact, a higher parameter measurement of rheology or crystallinity for a rPP material will potentially lead to better densification at a lower porosity a 3D printed object.^38–40

The evaluated geometric accuracy, a unique and the most important parameter used to define the quality of a 3D printed object, was practically acceptable for the GF/rPP composite materials recovered from non-woven waste masks. To achieve higher geometric accuracy performance of a 3D printed object, three important factors or mechanisms may possibly be involved in improvement of this parameter, as follows:
A strong adhesion force could be essential between the ﬁrst layer of 3D printing with GF/rPP composites and the printing platform (PP plate) in a 3D printing machine during its printing processing, where this type of adhesion force would primarily counteract the residual stress and warpage force of a 3D printed object during printing.

An inorganic skeleton might possibly exist due to the addition of GF material in a 3D printed object that was made from GF/rPP composites, where the GF additive mixed with the rPP polymer matrix may play a key role in hindering a possible contraction movement of the polymer molecular chain during the development of crystallization.^41–43

A lower value of the crystallinity parameter in a 3D printed GF/rPP composite object could be an advantage in decreasing the volume of shrinkage and warpage of a 3D printed object during the printing processing.

However, further effort in GF/rPP composites is required to improve their comprehensive performance, particularly in terms of the mechanical properties of a 3D printed object. In previous filament development efforts on some recycled thermoplastic wastes, it has been emphasized that proper additive selection at an optimized proportion applied to a thermoplastic polymer was critical during the double-screw modification process to fabricate its composite material.^44,45 In addition, more efforts and investigations in 3D printing parameter optimization and post-processing are also recommended to improve the relevant mechanical performance, as well as the geometrical accuracy for a 3D printed object from recycled polymer composites.⁴⁶

Currently, COVID-19 seems to have no end in the world due to the virus variants being updating and, accordingly, the daily generated waste masks in a large quantity will remain as environmental and health threat for a long time. To deal with this growing challenge along for non-woven waste masks and reduce their potential negative environmental impacts, some efforts have been made to recycle these waste masks in recent years, including pavement bases/subbases,^13,47 crude oil fluidity improvers,⁴⁸ textile supercapacitors,⁴⁹ etc. However, a simple, economic, sustainable, and high value-added approach is required. A novel recycling strategy for non-woven waste masks aiming to fabricate qualified 3D printing filaments was proposed and evaluated. With this unique design, this approach may possibly provide a feasible recycling option to deal with the large number of waste masks generated every day. In addition, the recycling strategy and its method we evaluated may possibly provide recycling technical guidance for other thermoplastic wastes in our life, not just non-woven waste masks, as a value-added 3D printing material.

Conclusion

In line with the growing challenge of a large quantity of non-woven waste masks during the unprecedented worldwide battle against the COVID-19 virus threat, a recycling strategy and method were proposed and evaluated in this investigation, where a GF/rPP composite filament was successfully developed using the recycled material from waste masks. Our investigations have demonstrated that the PP could be successfully extracted and recycled from non-woven waste masks and further serve as a 3D printing material. With a variety of evaluations designed in this investigation for the 3D printed object from GF/rPP composites, it has been shown that the GF additive at 30 wt% is an optimized and recommended proportion to mix with the rPP matrix in fabricating the GF/rPP composite filament for 3D printing processing. The specific warpage parameter (Wsp) and the surface roughness (Sa) for a 3D printed object from the GF/rPP composite at 30 wt% GF additive would indeed have printing accuracy of 0.54% ± 0.0014 and 21.1 ± 0.76 µm, respectively, and no clogging phenomenon was observed in the printer nozzle channel during the printing processing. A potential mechanism at break during the material property test as well as its high geometric accuracy for 3D printed object fabricated from GF/rPP composites was thoroughly analyzed via a XRD, SEM, and porosity testing. In addition, the different properties, such as densification, melting fluidity, crystallinity, and various mechanical properties, in a composite filament and its following 3D printed objects from GF/rPP and GF/PP composites were also illustrated and compared statistically. Clearly, our results have indicated that this recycling strategy and approach to deal with a large quantity of those disposable non-woven waste masks would be practically feasible and environmentally sustainable in developing a high value-added 3D printing product, which is not only beneficial to our environmental protection, but also of great assistance to maintain a sustainable method to battle with the COVID-19 threaten in the world.

Composite filament composition	GF volume fraction (%)	Theoretical density (g/cm³)
20 wt% GF/rPP^a	8.162	1.029
30 wt% GF/rPP^a	13.222	1.109
40 wt% GF/rPP^a	19.159	1.201
20 wt% GF/PP^b	8.526	0.985
30 wt% GF/PP^b	13.777	1.064
40 wt% GF/PP^b	19.907	1.156

Printing parameters	Value
Nozzle diameter (mm)	0.5
Nozzle temperature (°C)	200
Platform temperature (°C)	70
Layer thickness (mm)	0.2
Printing speed (mm/s)	50
Infill density (%)	100
Wall thickness (mm)	1
Infill pattern	Lines
Raster angle (°)	[–45, +45]
Overlap interval (mm)	0
Build platform type	PP plate
Build platform thicknesses (mm)	5

Filament composition	T_m (°C)	ΔH_m (J/g)	T_c (°C)	X_c (%)	T_d (°C)	T_v_max (°C)	Weight loss (%)
rPP^a	164.46	121.17	121.82	57.98	346.51	438.58	99.482
20 wt%GF/rPP^a	163.72	98.15	121.65	58.7	358.52	435.36	80.996
30 wt%GF/rPP^a	163.63	79.64	121.86	54.47	365.37	437.23	69.782
40 wt%GF/rPP^a	163.93	67.01	121.71	53.43	368.43	436.9	61.29
PP^b	150.68	87.84	119.87	42.02	319.59	417	99.878
20 wt%GF/PP^b	152.04	61.02	119.53	36.5	358.52	446	80.999
30 wt%GF/PP^b	151.05	53.91	118.85	36.87	365.88	441.6	72.236
40 wt%GF/PP^b	151.15	45.35	118.13	36.1	374.76	440.6	64.138

Mechanical properties at varying GF contents	GF/rPP*	GF/PP*	Commercial PP product*
Tensile strength (MPa)
20 wt%	19.06 ± 1.12^a	17.74 ± 0.12^b	17.17 ± 0.93^b,c
30 wt%	17.25 ± 0.41^bc	16.27 ± 0.16^bc
40 wt%	16.23 ± 0.32^c	14.83 ± 0.57^d
Elongation at break (%)
20 wt%	4.41 ± 0.19^c	23.18 ± 4.00^a	15.21 ± 2.66^b
30 wt%	3.79 ± 0.15^c	19.19 ± 3.04^a,b
40 wt%	3.32 ± 0.57^c	16.10 ± 0.85^b
Impact strength (MPa)
20 wt%	9.88 ± 1.27^b,c	15.78 ± 1.68^a	10.33 ± 1.05^b
30 wt%	7.31 ± 0.95^b,c	14.44 ± 1.43^a
40 wt%	4.19 ± 0.71^d	11.04 ± 1.60^b
Flexural strength (MPa)
20 wt%	26.51 ± 0.70^b	32.69 ± 1.47^a	17.94 ± 0.65^d
30 wt%	22.74 ± 0.48^c	30.8 ± 0.73^a
40 wt%	22.26 ± 1.66^c	20.89 ± 1.11^c
Compression strength (MPa)
20 wt%	32.85 ± 2.84^a	17.08 ± 0.78^c,d	19.86 ± 0.78^b,c
30 wt%	22.09 ± 3.87^b	15.44 ± 0.44^c
40 wt%	16.09 ± 1.21^c,d	14.92 ± 0.78^c

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Key R&D Program of China (2018YFE0107100) and a project funded by the Priority of Academic Program Development of Jiangsu Higher Education Institutions (PAPD 4013000011).

ORCID iDs

Jun Liu

Jianzhong Sun

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