Polylactic acid (PLA), REVODE190 (Zhejiang Hisun Biomaterials Co., Ltd., China), a 2D type with a specific gravity of 1.25 g/cm³, and thermoplastic polyurethane (TPU), WANTHANE® WHT-1570, a polyester-based grade with a Shore A hardness of 70 (Yantai Wanhua Polyurethanes Co., Ltd., China), were used in this study. PLA consists of over 99% L-lactide isomers [15–16], and TPU is a block copolymer composed of 31 wt% 4,4’-diphenylmethane diisocyanate, chain-extended with 1,4-butylene glycol (hard segment), and 69 wt% poly (1,4-butylene adipate) (soft segment) [17]. The glass transition temperature is reported as 40°C.

Carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH), designated US4305, were purchased from US Research Nanomaterials (US Nano). These CNTs have a carboxyl content of 2.56 wt%, an outer diameter of 5–15 nm, a specific surface area of approximately 230 m²/g, and a length of approximately 50 µm. Before melt mixing, both the polylactic acid (PLA) and thermoplastic polyurethane (TPU) pellets were dried in a vacuum oven at 50°C for 24 hours to remove any absorbed moisture.

The experimental procedure was conducted in two consecutive stages. In each stage, different samples were prepared by varying either the mixing sequence or the nanoparticle content.

In the first stage, an optimal melt-mixing protocol was identified to achieve uniform dispersion of carbon nanotubes (CNTs) within the polymer matrix. For this purpose, three different mixing sequences were evaluated. Initially, PLA/CNT and TPU/CNT masterbatches containing 4 wt% CNT were prepared using an internal mixer operating at 190°C and 60 rpm for 10 min. Subsequently, PLA/TPU/CNT nanocomposites were fabricated according to the selected mixing sequences described below.

In the second stage, nanocomposites containing different CNT loadings were prepared using the optimal mixing protocol selected based on the results of the first stage. The compositions of all prepared samples are summarized in Table 1.

Compression molding was employed to prepare sheets from each formulation. The samples were preheated for 3 min, followed by melting at 200°C under a pressure of 10 MPa for 5 min. The molded sheets were then rapidly cooled under a pressure of 20 MPa for 10 min, resulting in specimens with a nominal thickness of approximately 1 mm.

The general melt-mixing and compression molding procedures were carried out following previously reported methodologies, with appropriate modifications for the present system [2–3]

The thermal behavior of the samples was analyzed using a DSC (SD-L, Sanaf, Iran). Samples were initially heated to 200°C at a rate of 10°C/min and held for 3 minutes to eliminate thermal history. Subsequently, the samples were cooled to room temperature at a rate of 10°C/min and held for 3 minutes to examine the crystallization behavior. Remelting behavior was assessed by reheating the samples from room temperature to 200°C at 10°C/min. Melting temperature (T_m), glass transition temperature (T_g), and crystallization temperature (T_c) were recorded during the heating scans.

Tensile tests were performed using a Santam STM250 tensile testing machine in accordance with ASTM D882 standards. Rectangular samples were mounted between the machine’s grips with an initial gauge length of 50 mm, and a loading rate of 5 mm/min was applied. Tensile strength (MPa), elastic modulus (MPa), and elongation at break (%) were determined from the resulting stress-strain curves.

Samples with dimensions of approximately 2.9 x 12.8 x 64 mm were prepared for notched Izod impact testing. A 2 mm deep notch was introduced in the longitudinal direction using a SANTAM type BHO-CA splitter. The impact test was conducted using a SANTAM type SIT-20E device with a 20 J hammer, following the ASTM D256 standard. Three replicates were tested for each sample, and the average results were reported.

The viscoelastic properties of the samples were measured using an Anton Paar MCR-302 rheometer (Austria). Before testing, samples were dried at 50°C for 24 hours. The samples were then placed between two 25 mm diameter parallel plates with a 1 mm gap. Frequency sweep tests were performed at 190°C, within the linear viscoelastic region (1% shear strain), over a frequency range of 0.01 to 628 rad/s.

The morphology of the materials was investigated using a QUANTA 200 ESEM scanning electron microscope. Samples were fractured in liquid nitrogen. For the Set2 sample, the fractured surfaces were etched by immersion in dimethylformamide (DMF) solvent for 10 seconds to selectively remove the TPU phase. It should be noted that DMF is a solvent for both PLA and TPU, but TPU is more quickly affected by etching due to its higher solubility than PLA. Finally, the fracture cross-sections were coated with a thin layer of gold before imaging.

The Spectrodensitometer (Ihara S900, Japan) was used to test the compounds' optical density or printing quality. The Shimadzu BioSpec-1601 model UV-VIS spectrophotometer was used to determine the color properties (L*, a*, and b*) of the printed inks. A flexography printing simulator was used for the printing process. A Gretag Macbeth Colour Eye 7000A spectrophotometer (USA), which has 8/d geometry in a specular component included (SCI) mode, was used to measure the samples' color at 10-nm intervals between 380 and 780 nm. The results were then converted into CIELAB colorimetric coordinates (L*, a*, b*) using a CIE standard illuminant D65 and a CIE 1964 standard colorimetric observer. A rise in L* signifies that the sample is becoming lighter. A color shift towards red is indicated by a positive ∆a*, and a color shift towards green by a negative ∆a^*. Likewise, a color shift towards blue is indicated by a negative b^*, and a color shift towards yellow by a positive b^*. The rheological properties of the inks were studied using a US200 Anton Paar MCR 300 with a cone and plate geometry cell in controlled shear stress mode. In each case, the shear rate was increased from 0.01 to 10 Hz and reversed back down to 0.01 Hz, with 7 measurements distributed on a logarithmic scale for both increasing and decreasing shear rates.

Stress-strain curves of PLA, TPU, and a 75/25 blend of PLA/TPU were shown in Fig. 1, and in Table 2, tensile and impact test data were given. As can be seen in the figure and the values ​​shown in the table, the addition of TPU increased the elongation at break, toughness, and impact strength, and changed the failure behavior of the sample.

The PLA/TPU/CNT blends' DSC results for each of the three mixing methods were illustrated in Fig. 2 and Table 3. Although no significant difference was observed in the melting and crystallization behavior of the samples, as well as the values ​​obtained in this test, some details can be mentioned in this regard. Due to the slightly higher crystallization temperature (T_C) of S1, it was concluded that CNT makes a more effective contribution to the crystallization process. Furthermore, because the

The T_g of S1, S2, and S3 was 65.8°C, 67°C, and 67.8°C, respectively. The first method caused more CNT to be placed at the interface or PLA phase. In addition, this sample had the highest crystallization percentage (44.2%) compared to the others, which also confirms the aforementioned result.

The results of tensile testing on mixtures containing 1% by weight of nanoparticles and different mixing methods are displayed in Fig. 3 as a stress-strain curve. According to the image, the three samples all display similar tension-induced failure behavior, which suggests failure behavior that requires a lot of energy. Stress hardening occurs before failure, and cold stretching appeared after the yield point. However, the PT/1 S1 sample's tensile strength and Young's modulus were higher than those of the other two samples.

In this work, the different strategies or orders of material addition were examined to determine the possibility of identifying the location of nanoparticles and their impact on properties. To analyze the morphology of samples prepared in three different methods, scanning electron microscope (SEM) micrographs were taken. As illustrated in Fig. 4, the TPU areas are dispersed as spherical droplets in the PLA matrix as a dispersed phase. Although the three strategies have the same morphology, the first strategy showed the finer and more consistent droplets compared to the second and third methods (Note that here, no etching was used before taking SEM images).

Figure 5 shows the frequency sweep test results for mixes containing 1% by weight of carbon nanotubes created using different mixing methods. Even if there was no appreciable variation in the storage modulus values, the graphic demonstrates that there were significant changes in the mixing operations. The storage modulus in the S1 and S2 approaches showed a flat zone at low frequencies and an increase in value as the frequency increases.

While the S3 approach, in contrast to the other two strategies, essentially showed no flat region at low frequencies, this flat zone illustrates the formation of the physical network structure of carbon nanotubes as well as the shift from fluid to solid behavior. It was important to note that the S3 prototyping approach used the TPU/CNT masterbatch, indicating that the TPU masterbatch served as the main source of nanoparticles. The matrix, namely polylactic acid, determines the rheological behavior of the mixtures. The PLA matrix was more affected by the carbon nanotubes utilized in the S1 technique, as can be shown. This might be because the existence of the PLA matrix contains more carbon nanotubes. The loss modulus and viscosity were less affected by the mixing method.

The results of this section indicated that the morphology of all three methods was matrix drops with distributed TPU phases, considering that the first method (S1) had slightly smaller and more uniform droplets. Also tensile test demonstrated that S1 resulted in a higher tensile modulus, tensile strength, toughness, and elongation at the breaking point than other methods, which could be attributed to the remaining carbon nanotubes in the PLA phase or the interface of the two phases. The DSC test revealed that the S1 had a lower glass transition temperature and a higher degree of crystallization than the other methods, indicating that CNT plays a more effective function in compatibility and crystallization. The domain sweep test also revealed that in S1, nanoparticles had a bigger effect on the rheological properties of the PLA matrix, resulting in a higher storage modulus than in the other strategies. Given the results, the first mixing method was chosen as an appropriate mixing strategy for preparing nanocomposite samples.

Following the selection of the mixing method (S1), the impact of nanoparticle content was investigated. SEM micrographs of the produced nanocomposites are displayed in Fig. 6. In illustrated images, the holes are related to the TPU phase and parts of the PLA phase, because the PLA phase was affected less than the TPU phase by the etching. There are large, irregular holes in the PT/0 sample. When 0.2% carbon nanotubes are added, the pores significantly reduce in size and become more uniform. The size of the pores progressively rises when more CNT is added.

Tensile and impact tests were conducted to investigate how the content of CNT affected the mechanical properties of nanocomposites; in Fig. 7, the results of these tests are illustrated. The stress-strain curves of different nanocomposites are shown in Fig. 7a. The influence of CNT content on tensile stress, Young's modulus, toughness, and elongation at break is observed in Fig. 7b and c. Following the yield point, all samples displayed necking behavior, and before failure, stress hardening occurred.

The PT/0 combination has a tensile strength of 26 MPa, a Young's modulus of 1910 MPa, and an elongation at break of 166%. Tensile strength increased gradually with the addition of CNT up to 0.5% CNT; PT/0.5 reached 33 MPa, a 25% increase over PT/0. When nanoparticles interact favorably with the matrix, the tensile strength of polymer/nanoparticle systems rises [18]. However, tensile strength gradually decreased with values higher than 0.5%, reaching 25 MPa in PT/2, which is less than in PT/0. Additionally, the Young's modulus increased steadily with the presence of CNT, increasing by 15% to reach 2204 MPa with the addition of 1% of CNT. However, the Young's modulus unexpectedly decreased by 18% in PT/2. Its modulus is smaller than PT/0. Inaccurate nanoparticle dispersion and aggregation may be the cause of the decline in tensile strength and Young's modulus seen with high CNT levels. Furthermore, elongation at the breaking point of the combinations decreased steadily when CNT up to 1% by weight was present, but it was significantly decreased in PT/2. DSC curves for the first cycle of heating and cooling, also second heating of pure PLA, PLA/TPU (PT/0), and nanocomposites with different content of CNT were shown in Fig. 8. The test results were also reported in Table 4. As can be seen in Fig. 8(a) every sample shows an exothermic peak of cold crystallization; a large peak was formed by cold crystallization in pure PLA, demonstrating extremely slow crystallization dynamics in PLA. The cold crystallinity peak narrows and moves to a lower temperature when TPU and carbon nanotubes are present. The cold crystallization temperature (T_cc) and melting temperature (Tm) of pure PLA were lowered from 121.7°C and 182.8°C to 98.8°C and 181.5°C for PT/0 when blended with TPU, as Table 4 demonstrates. These values partially decrease as a result of carbon nanotubes. TPU's ability to accelerate crystallization was demonstrated by the considerable decrease in cold crystallization enthalpy (ΔH_cc) from 26.5 to 16.5. When carbon nanotubes were included, the cold crystallization enthalpy continued to decrease, reaching a minimum of 14.0 at PT/2. The degree of crystallinity (X_c) has increased from 1% in pure PLA to 21.1% in PT/0, which is also clearly visible. The presence of flexible TPU chains, which serve as softeners and facilitate the movement of rigid PLA chains, is the cause of this [19]. Additionally, the TPU areas act as the main nucleus for the production of crystals, enabling them to develop more quickly and at lower temperatures. Nucleation may occur as a result of hydrogen bonding between PLA chains and TPU hard sections [14]. Additionally, we noticed an increase when CNT was present in all samples. Additionally, the addition of CNT to all samples raised the degree of crystallinity; the PT/0.5 sample had the highest degree of crystallinity, increasing by 22% over PT/0 to 25.7%. The amount of dispersed phases, or TPU droplets, and the degree of crystallinity were both increased by the addition of carbon nanotubes. Figure 8(b) shows the crystallinity peak during cooling. As expected, the prepared mixtures show a higher crystallinity temperature (T_c) than pure PLA, but no significant difference is observed. The samples were heated a second time after the thermal history was removed. The DSC curve for the second heating is displayed in Fig. 8(c). After removing the thermal history, no cold crystallization was found in the other samples, despite the figure showing an exothermic peak of cold crystallization in the curve of pure PLA. The glass transition temperature (T_g) of the pure PLA sample is 64.7°C, according to Table 6, which was produced using the second heating curve. In contrast, the T_g of the PT/0 mixture, which has a glass transition temperature of 65.8°C, has increased in comparison to pure PLA. This indicates a lack of compatibility between the PLA and TPU phases. The glass transition temperature is slightly lowered by the addition of CNT, reaching 65.3°C at PT/0.2. The compatibility of the combination is unaffected noticeably by the inclusion of carbon nanotubes. Pure PLA has a 13.5% crystallinity once the thermal history is removed. Additionally, for the previously mentioned reasons, combining with TPU results in a notable increase in the degree of crystallization up to 39.2°C. The maximum degree of crystallization (47.6%) at PT/0.2 is achieved by adding CNT, which raises the degree of crystallization by 21% compared to the combination without CNT. The reason for the discrepancy in the degree of crystallization between the first and second heating cycles is that the samples cooled more slowly after the thermal history was removed than they did during the preparation of sheets from the samples and following compression molding.

The results of the amplitude sweep test with a constant strain of 1% are displayed in Fig. 9. The graph indicates a sharp increase in the storage modulus PT/0. The storage modulus (G') of nanocomposites is not considerably impacted by the addition of CNT in tiny amounts (less than 1% by weight). The behavior of the matrix, PLA, determines rheological characteristics like storage modulus in polymer blends. The placement of carbon nanotubes in the TPU phase can be responsible for slight variations in storage modulus.

A flat region and an increase in storage modulus are the outcomes of increasing the weight percentage of carbon nanotubes at low frequencies. Shi et al. found similar results, attributing this flat area in the storage modulus to the development of a carbon nanotube network structure in TPU phases [13], which delayed the relaxation of the TPU chain and caused a change in behavior from the liquid to the solid state. It is noted [20], the aggregation and synthesis of masses of nanoparticles, which restrict the long-range movements of the polymer chain, may also be the reason of the formed structure.

There is a similar trend in the loss modulus (G''). However, the impact on the storage modulus is more noticeable because of the stiffness and high elasticity of carbon nanotubes. The loss factor significantly decreases as the number of nanoparticles increases, substantiating this conclusion. Additionally, increasing the weight percentage of nanoparticles makes the complex more viscous; nevertheless, at low frequencies, the yield stress needed to break through the physical network of nanowires makes the viscosity even greater.

The ability of a substance's atoms to hold onto energy absorbed from an electromagnetic wave is indicated by its optical density. The optical density of the material through which electromagnetic waves pass determines how fast they move. I_0/I_1 = -log_10⁡ A(λ). Formula 4 1 states that I_0 is the intensity of light that is emitted, and I_1 is the intensity of light that is transmitted [21, 22]. Stronger optical density and better print quality are indicated by values for optical density that are closer to one [23].

Optical density is a measure of the film's ability to block light and absorb ink. Because they are nonpolar, polymers usually have a low surface energy and are not suited for printing. To improve printability and create prints of superior quality, significant surface energy is needed. increased surface polarity results in increased surface energy and higher-quality prints. Polylactic acid prints better than some polymers like PP and PET because of the ester groups in its structure, which make it more polar [24].

Optical density, contact angle, and CIE L*a*b* are compared before and after printing in Table 5. According to the contact angle test, CNT was added. The contact angle test shows that the addition of CNT raised the contact angle from 62 in PT/0 to 77 in PT/2, as shown in the table. Because of the carbon atoms in their graphene structure, nanoparticles maintain their hydrophobic properties even when they contain carboxyl groups in CNT. This means that adding them to PLA/TPU mixtures increases the hydrophobicity of the nanocomposite's surface. The table indicates that the optical density in PT/0 is 1.64 and that the optical density is not significantly affected by the addition of nani up to 0.5%. However, because the mixes were hydrophobic, the optical density decreased with higher CNT concentrations, i.e., 1 and 2% by weight. All of the mixes print well and have optical densities larger than one, but the PT/0.5 mix, which has an optical density of 1.65, yields superior results.

CIELab or CIE The International Commission on Illumination (CIE) created L*a*b*, a three-dimensional color space, to standardize and streamline color communication across all measurement devices. There are three axes in this color space [24]. The brightness of the sample is measured by the L* component; high numbers (51–100) indicate brightness, whereas low ones (0–50) indicate darkness. Red vs. green is the a* component, where green is represented by negative values and red by positive ones. When comparing yellow and blue, component b* uses positive values to represent yellow and negative numbers to represent blue. The elements of L*a*b* before and after color printing are displayed in Table 8 − 4.