1 Introduction

Thermoplastics like PVDF, polylactic acid (PLA), nylon 6, and ABS have wide engineering applications because of their high durability and cost-effectiveness. 3D printing of electrically conducting ABS-carbon black composite has been reported as a low-cost concave capacitor type polymeric sensor to measure the void fraction in a single and multi-component system in which two-phase flow takes place [1]. A highly conductive ABS-graphite-based electrochemical sensor for the detection of toxic gases has been prepared by a 3D printing process that highlights the industrial applications of ABS for sensor fabrication [2]. Some studies reported on circular ring-type patch antennas using conventional substrate materials like FR4, Roger, etc. for head imaging and 5G connectivity applications have also outlined the scope of thermoplastics for manufacturing cost-effective antennas [3, 4]. The review of the literature reported on antenna/sensor designs and applications has outlined that 3D printable composites of polytetrafluoroethylene (PTFE)-glass fiber, PVDF-polypyrrole-carbon nanotubes, and dielectric tuned PLA may be used as sensors because such composites possess acceptable conductivity and transducer properties for sensor applications [5,6,7,8]. The role of composite, thermoplastics, thermosetting, and meta-materials has been observed as very useful in recent studies for the design and development of patch antenna structures for signal transmission and microwave applications [9,10,11]. Fused deposition modeling (FDM) based 3D printing of ABS, nylon, and bakelite-low density polyethylene composites have been explored as an effective tool to fabricate complex dielectric structures from such materials for sensing devices [12,13,14]. Various surveys performed on manufacturing practices have highlighted the issue of plastic waste generation at a large scale during traditional and non-traditional manufacturing of plastic goods. Recycling of plastic solid waste (like PLA, ABS, PVDF, etc.) by mechanical/chemical-assisted mechanical processing for 3D printing applications has been widely reported [15,16,17,18]. The 4D properties in ABS-based composite and industrial uses of NH4Cl and ZnCl2 electrolytes have been reported for energy harvesting in nano-power generator devices and ESD. This indicates that such materials may also be used as sensors to monitor the health of ESD prepared by 3D printing processes [19,20,21,22,23]. Some studies outlined that the functional prototypes of ABS and its electro-active composites fabricated by single or multi-material 3D printing processes outlined good mechanical, structural, thermal, and electrical properties in ABS for one-way programmable sensing applications [24,25,26,27,28,29].

To ascertain the research gap, Scopus database of the past 15 years was explored for keywords 3D printed sensors, 3D printed energy storage devices, and correspondingly 3168, 328 results were obtained. For 3D printed sensors, online condition monitoring, and energy storage device no work has been reported. The reported literature as per the Scopus database was processed using VOSviewer open-source software for bibliographic analysis. For 3D printed sensors (3168 results), 44298 repetitive terms were observed. By selecting No. of repetitions as ‘5’ out of 44298 terms, 2159 met the threshold. For each of 2159 terms, a relevance score was calculated and 60% (1295) of the most relevant terms were selected for further analysis. Similarly for keyword 3D printed energy storage devices, (328 results), 15314 repetitive terms were observed. By selecting No. of repetitions as ‘5’ out of 15314 terms, 879 met the threshold. For each of 879 terms, a relevance score was calculated and 60% (527) of the most relevant terms were selected for further analysis. Figure 1a and b respectively show networking diagrams as bibliographic analysis for keywords 3D printed sensors and 3D printed energy storage devices.

Figure 1
figure 1

(a) Networking diagrams for keywords 3D printed sensors (b) 3D printed energy storage devices.

Full size image

Based on figure 1, it has been observed that little has been reported on online condition monitoring of ESD prepared by 2° recycling of ABS. This study reports the investigations on mechanical and electrical properties of electrolyte reinforced ABS composite (as 3D printed sensor) for online condition monitoring of ESD. In a typical dry cell, the electrolyte is one of the integral parts and the change in its dielectric properties with the time/ applied electric load has been used to ascertain the health of ESD (online) as the IoT-based solution (Bluetooth application) in ISM band (2.45 GHz). Based on the MFI test, 10% NH4Cl and 10% ZnCl2 (by weight %) were reinforced in ABS for preparing 3D printed rectangular substrates of a ring resonator for calculating εr and tanδ (for the resonant frequency of 2.45 GHz). The S21 parameters were observed using a VNA, and HFSS software package.

2 Materials and method

For the present work, recycled ABS granules, and electrolytes (NH4Cl, and ZnCl2) were procured from a local market to fabricate 3D printed sensors. Rheological analysis based upon MFI was performed for ABS and mechanically blended ABS-NH4Cl-ZnCl2 composite as per ASTM D 1238. Further analysis of mechanical, and electrical properties and bond characteristics was performed for selected composites. Figure 2 shows the work methodology adopted for the present study.

Figure 2
figure 2

Work methodology for 3D printed sensors.

Full size image

Based on acceptable mechanical and electrical properties, the FDM feedstock strands were prepared by twin-screw extrusion (TSE) and 3D printing of ABS and its composite substrates was performed. The dimensions of the substrate were designed using the HFSS software package and RF characteristics were obtained from VNA to ascertain the usefulness of the proposed composition/proportion for the desired health monitoring application.

3 Experimentation

3.1 Rheological analysis: MFI testing

Flowability of recycled ABS and different mechanical blended ABS-NH4Cl-ZnCl2 compositions/proportions for feasible 3D printing were investigated by the MFI test. The compositions weighing 50 g each were tested for MFI as per ASTM D 1238. The reinforcement of electrolytes was increased by weight % and tests were performed three times for each proportion, and average MFI was recorded. Table 1 shows the average MFI of primary (1°) and 2° recycled ABS compositions/proportions. It should be noted that an increase in electrolyte proportion, increases the probabilities of cross-linking in the ABS matrix, which may be responsible for reduced MFI. Further, the reduction in MFI of ABS composite may be due to an additional heating cycle, which the thermoplastic matrix has to withstand during 2° recycling. For commercially available open-source 3D printers, the range for MFI lies between 2 and 4 g/10 min (without any required alterations in hardware/software of the setup) [16, 18, 19, 24].

Table 1 Average MFI of 1° recycled ABS and 2° recycled ABS-NH4Cl-ZnCl2 composite.
Full size table

3.2 TSE

After ascertaining the flowability of 1° and 2° recycled ABS, feedstock filaments were extruded from a commercial TSE (Make: Thermo scientific). FDM feedstock of diameter 2.75 ± 0.04 mm was obtained for ABS and 80%ABS: 10%NH4Cl: 10%ZnCl2 composite as the same were observed acceptable for 3D printing of functional parts. The temperature of extrusion was 230°C, screw speed 100 rpm along with 5 kg applied load on both compositions/proportions was fixed based upon trial runs for smooth filament fabrication (visual observation).

3.3 Mechanical testing

A 90 ± 0.05 mm length of 1° and 2° recycled ABS FDM spools were tested for mechanical properties by performing their tensile test on a universal tensile tester (UTT/UTM) machine as per ASTM D638. The UTM test was performed to ascertain the mechanical strength in terms of Young’s modulus (E), Modulus of toughness (G), ultimate strength (σu), fracture strength (σf), etc. in the ABS and its composite for preparing durable products (useful for conformal devices and miniaturization).

3.4 3D printing, V-I, FTIR, and RF analysis

The V-I and FTIR-based electrical and bonding characterization of 1° and 2° recycled ABS was performed to investigate the effect of electrolytes on electrical conductivity and bond structure of 1° recycled ABS for acceptable sensor applications. Based on V-I and FTIR, the substrates of ABS and ABS-10%NH4Cl-10%ZnCl2 composite were 3D printed on an Ultimaker2+ FDM setup. The size of the 3D printed substrates was 50 × 40 × 0.75 mm. The 3D printing was performed at fixed settings: 45° raster angle, 100% infill density, infill pattern rectilinear, 03 number of the perimeter, 02 top and bottom layers. Table 2 shows the parameters selected for the FDM unit to 3D print the ABS and its composite-based substrates. Figure 3 shows the FDM setup (a), 3D printed substrate of ABS (b) and ABS-10%NH4Cl-10%ZnCl2 composite (c).

Table 2 FDM process parameters for 3D printing of ABS and its composite.
Full size table
Figure 3
figure 3

(a) Ultimaker2+ FDM setup (b) 3D printed 1°recycled ABS substrate (c) 2° recycled ABS composite substrate.

Full size image

3.5 Sensor design and simulation

Based on the ring resonator test reported in the literature [10, 27], the RF characteristics of 3D printed ABS substrates were observed. The observations for 1° and 2° recycled ABS composite substrate were utilized in the HFSS software package to design and simulate its online health monitoring-based sensor application for 3D printable ESD reported in previous studies [24]. The morphological analysis based on SEM-EDS and area mapping was performed to investigate the effect of electrolytes on improving recyclability and industrial applications of 2° recycled ABS.

4 Results and discussion

4.1 UTM results

The results obtained for average mechanical properties (like E, G, σu, and σf) of 03 samples are listed in table 3. It was observed that E and G improved for 2° recycled ABS composite filament samples (figure 4). Based upon figure 4, it has been observed that in the case of 2° recycled ABS, the mechanical properties have been reduced, thus making the functional prototype a little inferior in terms of mechanical strength, but the same may be used as a sensor (where mechanical strength is not the primary requirement), resulting in some value-added products.

Table 3 UTM results for 1° and 2° recycled ABS composite filaments.
Full size table
Figure 4
figure 4

Stress-strain graph (as per table 3).

Full size image

4.2 RF Characterization

The ring resonator test is being used for calculating εr and tanδ of the thermoplastic substrate. The change in dielectric properties of the substrate may help to ascertain the health of ESD. The RF characterization was performed using the microstrip ring resonator test. A ring resonator consists of a ring-type conductor material and transmission lines on the substrate used for determining the εr and tanδ of any material [10, 27]. The width for the feed line has been calculated for the impedance of 50 Ω. There is a small gap between the ring and feed lines ranging from 0.1 to 1.0 times the width of the feed line [10] The basic structure of the ring resonator is shown in figure 5 [10].

Figure 5
figure 5

Schematic of a ring resonator [10].

Full size image

The parameters used to design and analyze the patch antenna are shown in table 4.

Table 4 Dimensions of the ring resonator for ABS and ABS composite as substrate.
Full size table

The Cu-based conducting tape was used with a thickness of 0.08 mm. The prepared ring resonator samples are shown in figure 6.

Figure 6
figure 6

3D printed ABS ring resonator samples.

Full size image

Further, VNA was used for ascertaining the S21 parameters (using a range of 9 kHz to 6 GHz). It was observed that ABS resonated at 4.13 GHz with an insertion loss of −34.079 dB (figure 7a); while the ABS composite resonated at 2 peaks: 2.71 GHz (with a loss of −39.194 dB) and 5.44 GHz (with a loss of −33.055 dB) (figure 7b).

Figure 7
figure 7

S21 parameters of ring resonator (a) 1° recycled ABS, (b) 2° recycled ABS composite.

Full size image

Based upon S21 parameters, εr, and tanδ were calculated [10, 27] for 1° and 2° recycled ABS (table 5).

Table 5 Calculated εr, and tanδ.
Full size table

The antenna (sensor) was designed by keeping the resonating frequency of 2.4 GHz and the 1.6 mm thickness after the required calculations [10]. The calculated dimensions of the patch are shown in table 6.

Table 6 Calculated dimensions of the patch.
Full size table

The patch was designed and simulated in HFSS software for resonating at 2.4 GHz (figure 8).

Figure 8
figure 8figure 8

(a) 1° recycled ABS sensor design (b) simulated gain (c) simulated return loss (d)2° recycled ABS (e) simulated gain (f) simulated return loss.

Full size image

Based on figure 8, the simulations show (table 7) that both the patches resonate at 2.4 GHz with the1° and 2° recycled ABS. In other words, 1°/2° recycled ABS substrate has been suitably designed to give output at 2.4 GHz and it has been observed that in the case of 2° recycled ABS the patch size was marginally reduced. The same may be calibrated to estimate the remaining life/health of ESD. These are in line with other investigators [10, 27].

Table 7 Simulated results.
Full size table

4.3 V-I characteristics

The V-I characteristics of the samples were ascertained using Keithley 2400 Source Meter electrical tester apparatus (table 8). As observed in table 8, the resistance was significantly improved in the case of 2° recycled ABS (in line with εr, and tanδ, (table 5) hence may be used for sensor applications.

Table 8 V–I properties of 1° and 2° recycled ABS.
Full size table

4.4 Morphological analysis

The SEM and EDS were performed on 1° and 2° recycled ABS (figure 9). In 1° recycled ABS, EDS clearly shows a scattering of C and N on the surface which corresponds to its chemical compositions (C8H8·C4H6·C3H3N) as expected (figure 9a). As observed in figure 9b traces of Zn and Cl are also available.

Figure 9
figure 9

Surface features of (a) 1° recycled ABS (b) 2° recycled ABS.

Full size image

Further 3D rendered images, Ra profile, amplitude distribution function (ADF), and peak count (PC) were calculated based upon SEM images by using the Gwyddion open-source image processing tool (at cut-off length 0.05 mm) and Ra 25.97 µm and 28.9 µm was observed for 1° and 2° recycled ABS. The small variations in surface properties (such as the increase in Ra, ADF, and PC) in 2° recycled ABS have contributed to the change in resistance, εr, and tanδ of the substrate.

4.5 FTIR analysis

The absorbance spectrum obtained from FTIR for 1° and 2° recycled ABS compositions is shown in figure 10. The peak observed for C–H functional group at 2950 cm−1 wavenumber (WN) in 1° recycled ABS shifted sharply at the same WN for the 2° recycled ABS sample. At 2375WN, a sharp dip in the absorbance spectrum of ABS composite outlined the formation of a strong bond between the C≡N functional group and Cl present in the chemicals/salts. Also, the C=C bonds in 1° recycled ABS at 1489WN became strong as a shift was observed for 2° recycled ABS. The significant peak shifts for CH2 molecules at the 740–770 WN range also highlighted the same trend that contributed to the increase in mechanical, bonding, and electrical properties in 2° recycled ABS for sensing applications. These are in line with other investigators [19, 24, 27, 30].

Figure 10
figure 10

FTIR absorbance spectrum for(a)1° recycled ABS (b) 2° recycled ABS.

Full size image

5 Conclusions

Following are the conclusions from the present study:

  1. 1.

    Based upon the observed mechanical and electrical properties, the 3D printed sensor composed of electrolyte reinforced ABS composite may help for online condition monitoring of ESD. Hence it may contribute as a value-added functional prototype for improving the recyclability of thermoplastic waste.

  2. 2.

    The rheological analysis (based upon MFI) highlighted that 1° recycled ABS may be used effectively to fabricate composite-based 3D printed sensors by reinforcement of 10% ZnCl2 and 10% NH4Cl.

  3. 3.

    As regards mechanical, electrical, and bond structure properties are concerned, the ABS- ZnCl2- NH4Cl composite prepared by 2° recycling of ABS possesses acceptable E, G, V–I, and bond strength to impart durability to 3D printed sensor for online condition monitoring of ESD.

  4. 4.

    The RF properties outlined that the composite-based sensor may be calibrated to indicate the changes in the health of ESD with a variation of charge capacity in terms of εr, and tanδ.