Mechanically sustainable and primary recycled thermo-responsive ABS–PLA polymer composites for 4D printing applications: Fabrication and studies

3.1 MFI

MFI is one of the rheological properties that depict the fitness of the material to be used in the FFF-based 3D printing process. An MFI is a device or equipment that is used to measure the MFI. In the evaluation process of MFI, there is a weight applied to force the material to extrude through the die. According to ASTM standards D1238, the weight specified for the primary recycled ABS material is 5 kg at 200°C applied to a plunger, and the molten material is forced through the die. A timed extrudate (i.e., for 10 min) is collected and weighed. The SI units for MFI are g/10 min. Figure 3 shows the MFI setup and process.

Figure 3

(a) Sample placed in the barrel of MFI and (b) filament extruded through die.

3.2 Filament extrusion

The filaments were extruded in the form of 1.75 ± 0.1 mm cylindrical feedstock on a single-screw extruder setup (Felfil, maximum temperature: 300°C). The experimentation involves the control of screw rotation at 6 RPM (suggested by the manufacturer for ABS) and at a barrel temperature of 200°C. Before the extrusion process, the granules of primary recycled ABS and primary recycled PLA were dried in the hot air oven at 60°C for 1 h, and then extrusion was processed. The composition of the primary recycled PLA in primary recycled ABS varied from 5 to 40% (by weight %).

3.3 Tensile testing

Next to filament extrusion, the extruded material combination of ABS, PLA, and ABS–PLA composites was cut into 150 mm lengths and further subjected to tensile loading. A universal testing machine (Shanta Engineering, maximum capacity: 5,000 N) was used for the tensile testing of the prepared feedstock filaments at a strain rate of 20 mm·min⁻¹. It should be noted that the diameter of every sample was measured (repeated five times), and the mean values were used in the calculation. For the tensile testing of the feedstock filaments, there is no availability of set standards, so the ASTM D638 procedure was followed for the feedstock filament tensile testing. Before the tensile testing, the samples of feedstock filament were cut into the required length mechanically. After testing, SEM was performed further for fracture analysis of the fractured filaments.

3.4 Photomicrography

For the investigation of the morphology of fractured feedstock filaments, SEM (JEOL, JSM IT500) analysis was performed. For the investigations of the fractures, the power supply was maintained at 15 kV on a high vacuum mode. The microscopic fracture analysis was performed at magnifications of 75×, 250×, and 500×. In addition, the atomic and mass fractions of the atoms present were determined using the EDS software add-on. Optical microscopy (Nikon) was performed for fracture analysis at 50× magnification.

3.5 3D printing

The FFF-based 3D printer (Creality; Model: Ender 3 pro) was used to prepare the parts of primary recycled ABS, primarily recycled PLA, and ABS–PLA (which have better tensile strength) composites for the investigation of the SME. The 3D printing process was performed considering a layer thickness of 0.12 mm, a nozzle temperature for primary recycled PLA of 210°C, temperature for primary recycled ABS and ABS–PLA substrate (based upon the printability) of 230°C, bed temperature for the primary recycled PLA of 60°C, and temperature for primary recycled ABS and ABS–PLA of 80°C, a linear fill pattern, 80% infill, and a printing speed of 60 mm·s⁻¹. For the SME analysis, 3D printing was performed to prepare the structure in 40 mm × 10 mm × 1 mm size. Then, the slicing of that model was done using the slicing software Ultimaker Cura software package (Version 4.5).

3.6 Shape memory evaluation

These SMPs have high elastic deformation, as well as, in some cases, biocompatible, biodegradable, and cost-efficient. Therefore, the experiments were performed to check the SME of the part (flat strip), which was fabricated with the FFF technique, and the feedstock filament, which was manufactured by an extruder and used as a material. These SMPs can remember their original shape and come to their original shape even after deformation. Therefore, two terms exist: one is shape fixing, and another is shaped recovery ratio (RR). Shape fixing is defined as the level of deformation that may be fixed upon rapid cooling. Shape fixing can be calculated as shown in Eq. (1):

The shape RR is the level of deformation that is recovered upon heating and is calculated as shown in Eq. (2):

where l D is the length upon cooling after deformation, l 0 is the original length, l s is the length upon stretching at a specific temperature on which it is deformed, and l f is the final length after recovery [31].

The SME was investigated using a step-by-step procedure, as shown in Figure 4. After the fabrication of the flat strip, it was soaked in a beaker filled with water. The beaker was then placed in an oven at 70°C for 1 h. Subsequently, heating was enabled, and the part became flexible to some extent.

Figure 4

(a) Fabricated model of the flat strip. (b) Strip placed in a beaker of water at 70°C. (c) Cooled to room temperature and bent at an angle of 90°. (d) Placed in a beaker, and the temperature was increased to 70°C. (e) Occurrence of SME. (f) Retains its original position.

Next, the flat strip was subjected to hot air oven bending, which was performed at an angle of 90° followed by cooling at room temperature. After approximately 15 min, the flat strip was cooled and fixed at a 90° bending angle. Next, it was placed in a beaker of water and kept in an oven at 70°C for 1 h. In the final stage, the measurement of the angle was compared with the initial shape and size to calculate the SME.

4.1 MFI analysis

Table 2 shows the MFI value of the primary recycled ABS, primarily recycled PLA, and ABS–PLA composite. The experimentation was repeated three times for each composition to minimize the experimental errors, and the mean values of the three experiments are shown in Table 2. The MFI was observed at 11.85 and 45.0 g/10 min for primary recycled ABS and primary recycled PLA, respectively. The higher MFI for the primary recycled PLA shows that the PLA has a high tendency to flow through the FFF nozzle, which indicates that 3D printing may be done at a higher printing speed. Also, the addition of the PLA to the ABS matrix would significantly affect the MFI of the primary recycled ABS. The results suggest that increasing the amount of PLA in the ABS matrix leads to an increase in the MFI of the ABS–PLA composite matrix. Thus, the addition of the PLA in the ABS matrix increased the acceptability of the composite feedstock filaments. The MFI of ABS-40% PLA (31.48 g·min⁻¹) was highest among composites.

4.2 Tensile test results

The tensile testing resulted in tensile strength (ultimate and fracture tensile strength) and percentage elongation (at peak and break), as shown in Table 3. The ultimate tensile strength of the primary recycled ABS and primary recycled PLA was 19.49 and 26.58 MPa, respectively. Although the tensile strength of the primary recycled PLA was higher than that of the primary recycled ABS, the percentage of elongation was lower. The percentage elongation of ABS was 13%, which is greater than the percentage elongation of PLA (5%). The results have confirmed that PLA has higher stiffness than ABS. More interesting outcomes of mixing PLA and ABS were observed, as combining the tensile strength and percentage elongation of ABS–PLA was higher than either primary recycled ABS or primary recycled PLA alone. In the case of PLA loading of 20%, the maximum tensile strength (31.47 MPa) of ABS–PLA composites was observed. However, the percentage elongation of ABS-20% PLA was minimal, even lower than that of PLA. This may be because the blending of ABS and PLA was very compact, and the linkages between the polymeric matrices were unable to be established. Leading to this observation, the above loading of 20% PLA in ABS, the tensile strength started decreasing as the reinforcement of PLA was increased. This trend in the reduction of tensile strength was followed by a 40% loading of PLA in ABS. In the case of 25% loading of PLA in ABS (ABS–25% PLA), the percentage of elongation was observed to be the highest among all, even greater than ABS alone. This may be because 25% of the loading of PLA was unable to disturb the matrix of ABS, so the percentage of elongation resulted in the highest percentage in the case of ABS–25% PLA.

The modulus of toughness is one of the most important considerations when the manufactured parts are applied in an application where crash loading is involved. The higher modulus of toughness indicates the high resistivity of the manufactured parts against crash loading. Modulus of toughness results from the stress in correspondence with strain produced by the parts during tensile loading. Table 4 shows the modulus of toughness and elastic modulus of primary recycled ABS, primarily recycled PLA, and ABS–PLA composites. Although the elastic modulus of the ABS–20% PLA is maximum (786.75 MPa), at the same time, the strain is minimum in sample 6. Consequently, sample 6 cannot be used in the application where crash loading is required. Also, the strain is maximum for sample 7, but the elastic modulus for this sample is very low. Therefore, where crash loading is required, sample 7 can be used.

Figure 5 shows the stress vs strain graph of the feedstock filaments for fractured composites under tensile load. The curves depict that ABS–20% PLA possessed maximum tensile strength, whereas ABS–25% PLA possessed maximum elongation.

Figure 5

Stress vs strain curves for the fractured composites.

The experimental values in the present study were obtained in line with previous studies [32,33,34,35,36], as shown in Table 5. However, the mixing of engineered polymers, such as flax, may contribute to the maximization of the elastic modulus.

4.3 Fracture morphology

From the morphological test of the fractured filaments, various observations were made for different compositions of ABS-PLA composites (Figure 6). In ABS–5% PLA, the mixing of ABS-PLA layers was not uniform, elongation of the filament was also minimal, and there were non-mixing defects in this composition. In ABS–10% PLA, the mixing of ABS-PLA layers and elongation of filament were better than in the ABS–5% PLA composite. Subsequently, the mixing of the ABS-PLA layers was good to some extent, but the non-mixing defects occurred in ABS–15% PLA. The elongation of ABS–15% PLA was much better than that of ABS–5% PLA and ABS–10% PLA. In ABS–20% PLA, the mixing of the ABS-PLA layers was uniform with stretched edges, and also the elongation was maximum in all these samples, but the strain of ABS–20% PLA was minimal, so it cannot be used where crash loading is required. In ABS–25% PLA, the elongation was less than that in ABS–20% PLA, but the strain attained was maximum in all these samples; therefore, it can be used where crash loading is required. In ABS–30% PLA and ABS–35% PLA, the defects observed were like cavities formed due to non-mixing of the ABS-PLA layers, and somewhere these layers get separated from each other. Also, the elongation was not better than in other samples. In ABS–40% PLA, brittle fracture occurred, and the strain was also minimal.

Figure 6

Fracture morphological analysis of ABS–20% PLA and ABS–40% PLA filaments.

4.4 SEM analysis

Maximum and minimum tensile strengths were observed for ABS–20% PLA and ABS–40% PLA, respectively. Therefore, both combinations of materials were selected for fracture analysis using SEM and atomic fraction analysis using EDS. SEM analysis was performed after the morphological analysis of different compositions of ABS-PLA composites. From the SEM analysis, it was observed that the tensile strength in the ABS–20% PLA was better than that in all samples; however, minimum elongation was observed. Based on these results, it can be inferred that ABS–20% PLA is the most likely composition for applications that require high tensile strength.

The strain and modulus of toughness shown by ABS–20% PLA were very low, so they cannot be used in crash-loading applications. In ABS–40% PLA, the strain was better to some extent, and the mixing of the ABS-PLA layer was also better, but the brittle fracture occurred; therefore, this sample cannot be used for those applications where elongation is required. Figure 7 shows the ABS–20% PLA and ABS–40% PLA at various magnification values.

Figure 7

SEM analysis of samples of ABS–20% PLA and ABS–40% PLA filaments.

As shown in Figure 8, the carbon mass% in ABS–20% PLA is 33.68 ± 0.72, and the carbon mass% in ABS–40% PLA is 50.72 ± 1.05. In ABS–40% PLA, the brittle type of fracture that occurred may be due to more carbon content. The incorporation of carbon may lead to an increase in surface hardness as well.

Figure 8

EDS of samples of ABS–20% PLA and ABS–40% PLA composites.

4.5 Fracture profile

The fracture images of SEM were processed using the image analysis software tool (Gwyddion 2.60) to investigate the fracture profile and average surface roughness. It should be noted that there are more pores for the fracture of ABS–20% PLA, resulting in higher surface roughness (Ra = 210.1 nm). On the other hand, the presence of lesser pores in the case of ABS–40% PLA resulted in lower surface roughness (Ra = 122.0 nm) (Figure 9). In different aspects, it may be said that higher loading of the PLA in ABS may have contributed to filling the gaps in the defected spaces, reducing surface roughness. The experimental data show better mechanical properties for recycled ABS–20% PLA composites. In previous studies, it has been reported that with an increment in the weight proportion of ABS, the mechanical properties improved [32,36]. The increment in roughness lowers the strength of composites, which directly impacts the mechanical properties [37]. The incorporation of polymers by weight proportion affects the mechanical and morphological properties, which directly change the shape memory behaviors of the polymer composites. Previous studies have reported that the incorporation of fibers and particles significantly tunes the mechanical and morphological behavior of polymers and metals [38,39]. Sustainable composites have been manufactured by blending polymer compositions to attain better mechanical properties and can be used for their prospective applications [40,41,42].

Figure 9

Surface profilometry of ABS–20% PLA and ABS–40% PLA.

4.6 SME analysis

The highest tensile strength and elongation at fracture were observed in ABS–20% PLA and ABS–25% PLA, respectively. Therefore, along with the virgin ABS and virgin PLA, the SMEs of ABS–20% PLA and ABS–25% PLA 3D printed parts were also investigated (Table 6).

Table 6

SMEs of ABS, PLA, and ABS–20% PLA composites under a thermal stimulus

Materials	Initial angle	Bent angle	Angle recovered to
Primary recycled ABS
Primary recycled PLA
ABS–20% PLA
ABS–25% PLA

It was observed that primary recycled ABS has poor SME, as it did come to its original angle (180°) in the form of a flat surface. The primary recycled ABS returned to only 118°, which is very far from the original shape (angle = 180°). The 3D-printed primary recycled PLA exhibited a good tendency to memorize its shape when the effect of the stimulus was eliminated. The primary recycled PLA recovered to 176°, which is very near to its original shape at 180°. Most importantly, part 3D printed of ABS–20% PLA also recovered to 172°, exhibiting good shape memory tendency. Also, the shape memory of ABS–25% PLA recovered to 174°, which is also very near to neat PLA. Therefore, based on these facts, it can be concluded that the combination of ABS–PLA composites has good shape memory as well as better tensile properties.

Composite materials made of polymers such as ABS and PLA have attracted significant interest in the 4D printing industry. These composites are developed by blending PLA, a biodegradable polymer made from renewable resources like corn starch or sugarcane, with ABS, a thermoplastic polymer.

The resulting ABS–PLA composite material is incredibly adaptable and has numerous potential uses in 4D printing. With the aid of an emerging technology called 4D printing, materials can gradually alter their shape or other characteristics in response to environmental factors like temperature, humidity, or light.

Self-assembling structures are produced employing ABS–PLA composites in 4D printing. These materials can be programmed to change shape or form predetermined structures in response to certain stimuli. The use of self-assembling structures to build intricate machines or devices has potential applications in fields like robotics.

The formation of smart textiles is another use of ABS–PLA composites in 4D printing. The shape or properties of these materials can be programmed to adapt to changes in humidity or temperature. This may have implications for the fashion sector, where smart textiles might be used to make clothing that changes to fit the needs or environment of the wearer.

The versatility, biodegradability, potential for self-assembly, and smart functionality of ABS–PLA composites make them promising materials for 4D printing applications overall. We will probably see several more cutting-edge applications for this fascinating technology as the 4D printing industry develops. These materials provide an exceptional set of characteristics that make them perfect for designing intricate, responsive structures that can be tailored for a variety of applications.