Effect of zinc oxide suspension on the overall filler content of the PLA/ZnO composites and cPLA/ZnO composites

1 Introduction

Polylactic acid (PLA) is a polyester polymerized from lactic acid (LA) with excellent biocompatibility and biodegradability (1). PLA resembles polypropylene, polyethylene, and polystyrene in terms of properties (2). It can be produced using currently available production equipment (2). PLA is generated from LA, often present in plants and animals as a metabolic byproduct or intermediate product. LA and hydrolytic breakdown products of PLA are harmless by nature (1). PLA is biologically degradable by common microorganisms after hydrolysis. Furthermore, the ease with which PLA melts upon heating enables some unique applications in 3D printing.

However, the poor mechanical properties of PLA can be improved by incorporating carbon fibre into the raw material. Carbon fibre will aid in strengthening the PLA because of its excellent mechanical characteristics, corrosion resistance, and high specific strength and modulus (3). Carbon fibre-reinforced PLA composites have higher flexural strength, flexural modulus, and flexural toughness than pure PLA composites by 11.82%, 16.82%, and 21.86%, respectively, based on previous research (4). To guarantee that the complete characteristics of 3D-printed CF-reinforced composites are enhanced, the interfacial properties of 3D-printed thermoplastic composites must be improved (5). PLA carbon fibre is a combination of 85% PLA and 15% carbon fibre to produce tougher PLA parts. They reported that additive manufacturing technology with CF polymer composites improved the strength and stiffness of many components. According to particular research, when CF is treated with concentrated nitric acid, the surface roughness of the CF rises, which improves the mechanical interlock and interfacial adhesion between CF and matrix (4).

In addition, the major disadvantage of these pure polymer materials is that they have poor mechanical characteristics, which include low tensile strength and Young’s modulus (6). As a result, reinforcing fillers are necessary so that the composite material may combine the benefits of the polymer matrix and fillers into a single material that can be used in various applications. Adding particle reinforcement to the polymer material is one of the effective ways to increase the mechanical characteristics of each material. There are many types of reinforcement, such as metal particles, carbon/graphene fibres, metal oxides, organic fillers, and other forms. Metal oxides have been found to have excellent physical and mechanical properties, as well as being inexpensive and simple to synthesize (7,8).

Inorganic fillers act as rigid fillers and nucleating agents, especially during annealing, to increase the crystallization of PLA and thus cause the modulus, tensile strength, and crystallinity to be higher (9). Controlling the ratio of two kinds of PLA is also an effective way to monitor the degradation of PLA (9). In addition, incorporating nanofillers, such as organo-modified layered silicates, silver, zinc oxide (ZnO), graphite derivatives, carbon nanotubes (CNTs), and silica, is a realistic and feasible approach for improving the antibacterial, barrier, thermal, and mechanical properties of PLA (9). However, the cost of silver restricts the application. Metal oxides like ZnO and TiO₂ are widely used in health-oriented applications due to their cost-effectiveness, versatility, and special physicochemical features (9).

Additive manufacturing processes, such as 3D printing, are based on computer-aided data sources and have resulted in tremendous convenience to technology today. Additive manufacturing attracts the attention of researchers and engineers in many fields, especially in medical applications, aerospace, spare part products, and others. The reasons are due to fast design, and final geometrical components can be produced in a short running time. Material extrusion-based additive manufacturing is especially suited for printing electrical components using various materials at a low cost. Additional machining processes are reduced or skipped entirely, while final costs and material waste are significantly reduced. The most prevalent approach in 3D printing is fused deposition modelling (FDM), which primarily uses a polymer filament (10). The filament will be heated at the extruder nozzle to ensure that the filament’s plastic composite is in a semi-liquid condition. After that, the filament is extruded onto the platform or over previously printed layers. The thermoplastic nature of the polymer filament is critical for this process, as it allows the filaments to fuse during printing and then solidify at ambient temperature immediately (11). FDM builds a cross-sectional layer of extruded filament to form 3D geometry (12). Usually, materials used involve PLA, ABS, and polyetherimide.

This research aims to improve the properties of PLA reinforced with a conductive ZnO reinforcement filler produced by additive manufacturing. ZnO belongs to the class of transparent conducting oxides, which can be used as transparent electrodes in electronic devices (13). The RepRap Mendelmax 1.5 FDM 3D printer fabricated PLA and cPLA polymer composites. The impact of using precoated ZnO fillers was significant in this study. The performance of the composites was investigated using thermogravimetric analysis (TGA), FTIR, tensile testing, scanning electron microscopy (SEM) inspection, dynamic mechanical analysis, electrical impedance testing, and thermal conductivity testing.

2 Materials and methods

3 Results and discussion

TGA determined the ZnO suspension content (wt%) inside produced PLA composites. Figure 1 illustrates the thermograms of specimens printed at varied dispenser speeds, which were 1,000, 1,400, and 1,800 rpm.

Figure 1

Thermograms for PLA and cPLA composites printing with different dispenser speeds.

The TGA curve revealed that PLA/ZnO and cPLA/ZnO decompose in a single stage and two stages, respectively. The deterioration of PLA began at about 270°C, as seen by the weight loss. The decomposition process was completed at almost 500°C with the remaining residue mass indicating the amount of ZnO filler incorporated into composites. Based on Figure 1, 11 wt% ZnO filler was dispensed onto the PLA with the high dispenser speed. During the printing procedure, 9 wt% and 7 wt% ZnO were dispensed on the composites at 1,400 and 1,000 rpm dispenser speeds, respectively. For cPLA, the thermal decomposition began at 290°C with a total mass loss of 60%. Switching the gas from nitrogen to oxygen at approximately 380°C resulted in carbon oxidation. When the dispenser speed was increased to medium or high, the filler amount was increased to 9.33 and 11 wt%, respectively. This demonstrated that the filler content could be regulated by adjusting the dispenser’s motor speeds.

FTIR analysis is a technique to detect the presence of functional groups across the infrared spectrum of the PLA/ZnO composites. All the samples had almost identical FTIR spectra, implying that they have the same chemical composition and structure.

Figure 2 shows the structural comparison of pure PLA, cPLA, PLA/ZnO, and cPLA/ZnO composite spectra. The first peak found for both PLA samples and composites ranged between 3,577 and 3,700 cm⁻¹, and the second peak found fell in the range of 2,900–2,950 cm⁻¹. These first peaks indicate the presence of hydroxyl O–H stretching, while the second peaks are allocated to the stretching of C–H in both pure samples and composites. The 2,559.29 cm⁻¹ region band reflects the hydroxyl group by absorbing hydrogen on ZnO. There were peaks found at 2,012.44 and 2,171.11 cm⁻¹ wavelengths for both samples, respectively, which were assigned to C≡C stretching, while the strong peaks for PLA located at 1,713 cm⁻¹ range indicated the presence of C═O stretching vibration of the carbonyl group for the ester unit (14).

Figure 2

FTIR spectra of pure PLA, cPLA, PLA and cPLA composites.

Next, the peaks at 1,451.59 and 1,364.60 cm⁻¹ for pure PLA and PLA/ZnO composites were assigned to C–H deformation, which is symmetric and asymmetric bending, respectively. Additionally, the spectrum in the region of 1,080–1,260 cm⁻¹ indicated the presence of strong C–O–C stretching (14). A huge shift occurred for PLA at 1,713 and 1,250 cm⁻¹ due to the absence of ZnO reinforcement. The peak decrease for composites could be attributed to the interaction of ZnO in the pure PLA polymer matrix. Since ZnO accounts for a small part of the composites, its normal peaks are not as prominent as shown in Figure 2 (15). According to Figure 2, the absorption band at about 860 cm⁻¹ corresponding to the C–C stretching showed evidence of amorphous phases of PLA contributing to the low density or disappearance of peaks (16). Overall, the FTIR spectra for the composites in the studied range practically revealed that the intensity of ZnO reinforcement did not change in any peaks in the PLA except for the peak at 3,450 cm⁻¹ and assigned formation of secondary forces such as weak hydrogen bonds between the PLA polymer and ZnO (17). All PLA/ZnO composites exhibited a similar band to pure PLA, with a less noticeable change in the peak area.

Figure 3 shows the effect of different ZnO loadings on the tensile strength of PLA and cPLA composites. The tensile strength increased progressively for PLA and slightly increased for cPLA as the filler loading increased, as seen in the graph. The tensile strength increased from 29.12 MPa to a maximum value of 39.50 MPa when filler loading increased to 11 wt%. The results revealed that the ZnO mixed well in PLA, implying that good interfacial adhesion will facilitate effective stress transfer. This is owing to the tensile load being transferred to the filler before it fails (18).

Figure 3

Effect of ZnO loading on tensile strengths of PLA/ZnO and cPLA/ZnO composites.

However, pure PLA and the cPLA control sample have low tensile strength compared to all the composites. This illustrates ductility behaviour because the pure samples had minimal stress to fracture the samples. The tensile strength of the cPLA composite is significantly lower than that of PLA. This is because of the weak bonding between carbon, PLA, and ZnO. Initially, the carbon fibre dispersed well into the PLA filament. The cPLA filament melted at the same melting temperature as the original PLA during printing. It will develop the cluster formation of carbon when PLA is in the molten state and results in carbon fibre that is no longer uniformly dispersed (18). Adding ZnO will hinder cluster attachment and finally cause secondary agglomeration (19). There was a slight increase in tensile strength when the ZnO loading increased to 9 wt% and dropped when it reached 11 wt%. This is because when ZnO filler reaches a high population, non-homogeneous ZnO induces the lump and agglomeration making stress transmission from matrix to fibre complicated and disrupting the matrix phase’s continuity (20). This was proven by the SEM micrograph, as shown in Figure 4.

Figure 4

(a) SEM micrographs of the pure PLA sample, (b) PLA sample with 4 wt% ZnO, (c) PLA sample with 7 wt% ZnO, (d) PLA sample with 11 wt% ZnO, (e) pure cPLA sample, (f) cPLA sample with 4 wt% ZnO, (g) cPLA sample with 7 wt% ZnO, and (h) cPLA sample with 11 wt% ZnO.

Figure 4b and c shows the SEM micrograph of PLA printed with 7 and 9 wt% ZnO fillers. ZnO fillers diffused effectively inside the PLA matrix and produced adhesion with the PLA interface, as demonstrated in Figure 4d. There was an increase in the tensile strength for all composites filled with ZnO fillers compared to the unfilled control samples (Figure 4a). When the filler loading was increased to 9 wt%, the tensile strength decreased by 1.5%. This is because when filler loading was increased to a critical level, non-homogeneous ZnO fillers are expected to induce agglomerates, which generate a stress concentration zone that causes cracking and finally weakens the tensile strength of PLA composites (21). The PLA reinforced with 11 wt% ZnO filler achieved the maximum tensile strength due to the strengthening impact of the filler on the composites, as shown in Figure 4d (22).

Based on previous studies, Alig et al. investigated the effect of filler arrangement and agglomeration on the mechanical, electrical, and thermal characteristics of a CNT-filled composite (23). Due to poor dispersion in the polymer matrix, first or primary agglomerates developed during the CNT manufacturing process. Secondary agglomerates (agglomerates formed as a result of secondary processing, such as melt mixing, extrusion, or injection moulding) formed inside the composites due to non-Brownian diffusion of particles having attractive interactions. The tensile characteristics degraded by the presence of secondary agglomerates that were more loosely packed (23). Initial agglomerates formed during the printing process when PLA was heated to its melting temperature, resulting in poor carbon black (CB) dispersion inside PLA. When the ZnO filler solution was applied, there will be a lack of bonding because the temperature decrease leads to a loss in tensile strength. The SEM micrograph showed the formation of agglomerates in Figure 4h, while Figure 4e illustrates well dispersion of CB in PLA samples.

Elemental mapping is required to understand the distribution of fillers and the concurrence with the explanation given previously. The presence of ZnO filler distribution was confirmed in composites, as shown in Figure 5c and d.

Figure 5

(a) Elemental mapping of pure PLA sample, (b) pure cPLA sample, (c) PLA sample with 11 wt% ZnO, (d) cPLA sample with 4 wt% ZnO.

Figure 6 shows the effect of ZnO loading on the tensile modulus of the PLA and cPLA composites. Filled PLA had a greater tensile modulus than unfilled PLA. When the filler content increased, the tensile modulus increased from 1,247.18 to 1,578.08 MPa. At 11 wt% ZnO loading, the maximum value of 1.58 GPa was achieved. The tensile modulus of cPLA composites demonstrated a similar trend to PLA due to the stiffening role of fillers. Fillers functioned as obstacles that restricted the polymer chains from moving freely and therefore reduced the flexibility of the composites (18). Filler rigidity increased the modulus and contributed to the ability to tolerate deformation. Furthermore, the cPLA composites had lower tensile modulus than the PLA composites with the same filler amount. This may be due to the carbon fibre creating the uneven arrangement of clusters resulting in the loss of adhesion between the reinforcer and matrix, thus reflecting the reduced stiffness (24).

Figure 6

Effect of ZnO loading on Young’s modulus of PLA/ZnO and cPLA/ZnO composites.

Kumar et al. studied the effects of alumina filler concentration on Young’s modulus of the ABS/alumina composite (25). The alumina loading varied from 1 to 5 vol% within ABS. At a filler loading of 5 vol%, the modulus increased by 9% compared to that of pure ABS. The concentration of particles filled between the polymer chain was much higher and thus the polymer chain segment would deform itself in a less mobile manner, accounting for the higher Young’s modulus. The results were similar to the experimental results, which showed that with 11 wt% loading, the moduli of PLA/ZnO and cPLA/ZnO increased by 27% and 18%, respectively (26).

Figure 7 demonstrates the effect of different ZnO loadings on the dynamic storage moduli, E′, of PLA and cPLA composites. The storage modulus reduced dramatically when temperature increased, and there was a significant drop in the T _g region. At the same temperatures, the storage moduli of PLA and cPLA composites were greater than those of pure PLA and cPLA. At all temperatures, the storage moduli increased as the filler loading increased, and the composite with the highest filler loading, PLA and cPLA reinforced with 11 wt% ZnO, had the highest E′ value. This was linked to the inclusion of stiffness improvement with the filler addition (27). The fillers reinforcing impact and strong matrix/filler interaction boosted the composites’ overall rigidity (20). This inhibited the mobility of molecular chains of the matrix, thus increasing the stiffness of the composite materials (28).

Figure 7

Dynamic storage moduli (E′) of PLA/ZnO and cPLA/ZnO.

However, cPLA/ZnO composites had a lower E′ than PLA/ZnO composites due to the presence of CB and ZnO clusters, which lowered the material’s stiffness. According to previous studies, Santos et al. discovered a similar result when they investigated the characteristics of ABS reinforced with titanium dioxide (TiO₂) and CB. The agglomerate formation was found to be a fault that caused stiffness deterioration. Because of the inadequate interaction between CB and TiO₂ in the ABS matrix, a better degree of stress transfer at the boundary became impossible (29).

Figure 8 displays the effect of different ZnO loadings on the dynamic loss moduli, E″, of PLA and cPLA composites. The curve representing the loss modulus remains constant at low temperatures and increases gradually until reaching the peak value because intermolecular bonding is destroyed (30). It was observed that the loss modulus decreased continuously after the highest peak as the mobility of the chain improved. Owing to the decrease in stiffness, more energy is dissipated; therefore, the highest loss modulus is observed for less rigid samples (31). The reduction in the intensity of the matrix chains’ motion is associated with an increase in T _g. At the same temperature, the loss modulus of composites with increasing filler loading was lower. This might be owing to the polymer chain’s constrained mobility, which results in less energy being dissipated due to the chain restriction. The result showed that the loss modulus tends to lose energy, which is inversely proportional to the storage modulus (31).

Figure 8

Loss moduli (E″) of PLA/ZnO and cPLA/ZnO composites.

Figure 9 shows the increasing trend in thermal conductivity when the ZnO loading increased from 7 to 11 wt% for both composites. This is owing to the use of a filler with a higher thermal conductivity than the PLA matrix, which causes higher heat dissipation between the ZnO filler and the PLA matrix. The maximum thermal conductivity was attained by 11 wt% cPLA, which was 0.3074 W‧mK⁻¹, while the lowest reading was acquired by pure PLA, which was 0.1412 W‧mK⁻¹. Because the matrix acts as a thermal barrier for heat propagation, more fillers will mix well with PLA and produce conductive pathways to transport heat rapidly (32). As we can see, when the ZnO filler is increased, the thermal conductivity of PLA composites increases from 0.1962 to 0.2361 W‧mK⁻¹, indicating that more filler particles are packed together to generate effective close contact between them and form conductive pathways (33). Due to naturally high thermal conductivity CB, the thermal conductivity of cPLA composites was greater than that of PLA composites (34). Fillers are well dispersed inside the polymer matrix at 7 and 9 wt%. Some fillers developed agglomerates at larger 11 wt% filler loadings, promoting quick heat conduction and increasing thermal conductivity at 0.3074 W‧mK⁻¹ (32). This can be proved through SEM micrographs Figure 4h reveals the generation of agglomerates inside the composites. This is similar to the result of previous studies, which found that CB particles formed conductive clusters and networks in the poly(ethylene-oxide) polymer matrix when the sample approached the percolation threshold, therefore improving thermal conductivity (35).

Figure 9

Thermal conductivity of PLA and cPLA composites.

Figure 10 shows a comparison of the impedance testing of PLA/ZnO and cPLA/ZnO composites. The 7 wt% of cPLA composite had the highest impedance at 10 kHz frequency. This is due to the CB component in cPLA. The electrical conductivity of PLA/ZnO and cPLA/ZnO composites steadily enhanced with the inclusion of ZnO fillers. When the fillers achieved their percolation threshold value, conductive networks of ZnO formed within the matrix (36). CB and ZnO filler generated a high electrically conductive network, facilitating carrier mobility inside the polymer chains and hence decreasing the impedance of materials (37). The previous research stated that PLA has low permittivity due to its small polarization of the macromolecules (38). Among the polymer, epoxy was used due to its high impedance properties. However, epoxy has low toughness. Previous research stated that the impedance in zinc-rich epoxy was 80 Ω‧cm² at 100,000 Hz (39). All PLA/ZnO and cPLA/ZnO composites had lower impedance when frequency increased. Therefore, cPLA/ZnO was suitable for replacing epoxy composites. According to Abdalhadi et al., the matrix of OPEFB/PLA with Fe₂O₃ filler was homogeneous and the number of voids was reduced (38). Therefore, suitable ZnO suspension content was also an important factor that could influence the properties of composites.

Figure 10

Impedance testing of PLA/ZnO and cPLA/ZnO composites.

4 Conclusions

This study shows that the 11 wt% PLA/ZnO composites have the highest mechanical properties which are tensile strength and Young’s modulus. According to the results, the mechanical properties of the samples with filler addition were greater than those of the pure samples. These enhancements were made possible by improving the interfacial adhesion between the PLA matrix and the ZnO suspension. Because of the stiffening effect and high strength of the ZnO suspension, tensile strength and Young’s modulus were improved by incorporating ZnO into the PLA matrix. The detailed morphology can be achieved by SEM analysis due to the uniform diffusion of ZnO in the PLA, indicating excellent interfacial adhesion that will facilitate effective stress transfer. This eventually leads to high tensile strength and dynamic mechanical properties. Because of the existence of CB and ZnO clusters, cPLA/ZnO composites have a lower E′ but higher E′′ than PLA/ZnO composites. In addition, the inherent high thermal and impedance of cPLA/ZnO compared to those of pure and PLA composites is due to the incorporation of the carbon fibre and conductive ZnO into PLA. This is because the formation of a conductive path between them transmits heat and electricity quickly. Our approach expands the good mechanical, electrical, and thermal conductivity performances by integrating the ZnO suspension into PLA to provide superior-quality electrical instruments for the future.