*2.4. Microscopy*

Figures 4 and 5 present the outer and inner texture of 3D-printed (Figure 4) and injection-moulded samples (Figure 5) under a digital light microscope and SEM with an additional EDS analysis (Figure 4e). For the printed samples, different morphologies of the outer and inner regions of the fractured sample (Figure 4b,c) can be seen due to the printing pattern, as the infill has a grid pattern for improved mechanical strength, while the outer layers are made in a rectangular pattern and therefore can be seen as perpendicular to the fracture plane. This is a result of the standard printing process conditions, where the 3D printer outlines the outer shapes of the sample with straight lines first, and then infills the object with a chosen pattern. However, the outline of the outer layers is, in fact, of the highest interest, as it allows observation of the inter-layer interfaces between the individual extrudate strands (and, on this basis, an assessment of the layer-to-layer bonding), as well as the size of the air gaps. In Figure 4b, the inter-layer interfaces between extrudate strands of the neat PLA sample can be clearly seen, which contributes to the low mechanical resistance of the printed PLA objects. On the other hand, for 0.25% SS-Limonene/PLA sample (Figure 4c), this interface is hardly visible, which explains the improved inter-layer adhesion and mechanical rigidity of the samples, which made them mechanically more similar to the injection-moulded ones. However, the formation of air gaps was rather unaffected. Also, on closer inspection, all the samples containing SS-Limonene additive (either 3D printed or injection moulded) contained particles visible both under the light microscope (Figure 4d) and SEM (Figure 4f,g and Figure 5d,e). EDS analysis of silicon allowed identification of the particles to be agglomerates of the polymerized additive (Figure 4e). With increasing loading, more agglomerates of such particles were visible, which contributed to the issues with printing the samples containing high amounts of SS-Limonene. Polymerization of the additive is further discussed in Section 2.5. Additionally, for injection-moulded samples, together with the mentioned particles, small air gaps were observed (Figure 5d,e), which are not present in the sample made from neat PLA (Figure 5b,c). It may be due to difficulties with degassing of the polymer melt, or the generation of gas products either of evaporation or decomposition of SS-Limonene, as the thermogravimetric analysis thereof under the PLA processing temperatures revealed a small mass loss.

**Figure 4.** Optical microscopic images (**<sup>a</sup>**–**d**) and SEM images (**<sup>e</sup>**–**g**) of printed samples: outer surface (**a**), fractured sample from neat PLA (**b**), fractured sample from 0.25% SS-Limonene/PLA (**c**), a crystal of polymerized SS-Limonene (**d**), silicon EDS image of 0.25% SS-Limonene/PLA fractured sample (**e**), fractured sample from 0.25% SS-Limonene/PLA (**f**), fractured sample from 2.5% SS-Limonene/PLA (**g**).

**Figure 5.** Optical microscopic image (**a**) and SEM images (**b**–**<sup>e</sup>**) of injection moulded samples: outer surface (**a**), fractured sample from neat PLA (**b**,**<sup>c</sup>**), fractured sample from 2.5% SS-Limonene/PLA (**d**,**<sup>e</sup>**).

#### *2.5. Thermal Analysis Results*

Thermal effects for SS-Limonene/PLA compositions were measured by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

DSC analysis was performed to determine the effect of SS-Limonene addition on the glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures of the composites. The graphs are shown in Figure 6. In each case, a large peak of glass transition is noticeable at the first heating cycle, which is related to the low crystallinity of the extruded samples (high proportion of the amorphous phase), due to the rapid cooling of the polymer (in air and in a cooling bath during extrusion). The glass transition temperature (Tg) after the second heating cycle of the test samples containing the organosilicon additive is shifted towards lower values relative to that observed for the neat PLA. This is due to the plasticization of the polymer matrix. Similar conclusions can be drawn in the context of Tcc, as the presence of the plasticizing phase increases the freedom of the chains in the amorphous phase, accelerating the initiation of crystallization. The change in Tm and Tcc values also shows that SS-Limonene is at least partially mixed with PLA and the interaction of these two components occurs, despite the presence of the polymerized additive phase visible in the microscopic photos (SEM-EDS, light microscopy). On the basis of Tg, the strongest plasticizing effect was observed for the system containing 0.25% of the above-mentioned modifier, but the lowest melting point was observed for the

system containing 5% of SS-Limonene. The DSC analysis of the modifier allowed observation of the polymerization of the compound on heating of the sample, which was also confirmed by microscopic analysis as an effect of agglomeration of the polymerized additive in the PLA matrix. A similar effect has been observed in our earlier work, however, most likely due to lower processing temperatures of LDPE, the agglomeration was far less severe [31].

**Figure 6.** DSC curves recorded for samples of SS-Limonene/PLA composite.

All results of the TGA, DTG and c-DTA measurements are presented in Figure 7. The parameters determined, including the temperature of 1% mass loss, onset, and temperature at the maximum rate of mass loss are collected in Table 2. The process of thermal decomposition of samples was carried out in both nitrogen and air atmosphere. It should be remembered that thermal changes in thermoplastic systems at temperatures above the melting point of the matrix should not be defined as the 'thermal resistance' or 'thermal strength' of the composite, but refer to the influence of the applied additives on the thermal stability of the polymer system in molten state and the interaction between the system components in the melt. Based on the observation of the complete thermal analysis, i.e., the derivatographic curve, DTG and c-DTA results obtained both in air and nitrogen atmosphere, three stages of thermal transformation can be distinguished. The first step, observed in both air and nitrogen atmosphere, at 140–160 ◦C is related to the polymerization of SS-Limonene. The second stage, taking place in a nitrogen atmosphere at a temperature above 365.7 ◦C, is related to the cracking of PLA chains with the simultaneous endothermic distillation of mers, including SS-Limonene degradation products and lactides. According to the c-DTA curve determined in the air atmosphere, the endothermic distillation process overlaps with the exothermic oxidation process of the cracking products. In the last, third stage, in the air atmosphere, one more signal is observed at a temperature of 550–700 ◦C, which characterizes the process of burning coke, originating mainly from the organosilicon derivative. Stage 1 refers to the functional properties of the composite material, while stages 2 and 3 describe the irreversible processes of thermal decomposition of a mixture of molten PLA and an organosilicon derivative.

**Figure 7.** TGA, c-DTA and DTG curves of SS-Limonene/PLA composite in (**a**) air and (**b**) nitrogen atmosphere.


**Table 2.** Results of thermogravimetric analysis.

#### *2.6. Contact Angle Measurements*

The contact angle measurements were performed for SS-Limonene/PLA composites obtained by two different methods—FDM and injection moulding. For the measurements of 3D-printed samples, the samples were placed with the layer-by-layer deposition plane oriented parallelly to the plane of the goniometer stage. The contact angle of the neat PLA was 81.4◦ for the printed samples and 83.6◦ for the samples obtained in the injection process, in both cases the surface before modification showed hydrophilic properties. Modification of PLA with SS-Limonene increased the hydrophobicity of all the tested samples (see Table 3). For the printed samples, regardless of the modifier concentration, a hydrophobic surface effect was obtained (the value of the contact angle increased to above 90◦). For the samples obtained by injection moulding, the increase in the value of the contact angle was insignificant and it remained at a similar level (max. by 4.3◦). The difference in the degree of hydrophobicity of printed and injected samples is due to their surface structure and microstructure. 3D printing allows obtaining microstructures and surface roughness, which results in higher values of the contact angle. This thesis was confirmed by microscopic observations (see Section 2.4). On the other hand, injection moulding produces more smooth surfaces (if no modification of the mould surface is applied), which reduces the microstructure effect on the surface and therefore almost no effect of the organosilicon additive can be observed.


**Table 3.** Water contact angle [◦].

#### *2.7. Mechanical Properties*

#### 2.7.1. Tensile Strength and Flexural Strength

Mechanical tests were carried out for the modified samples obtained by both 3D printing and traditional injection moulding. For the 3D-printed samples, the tensile load was applied parallelly to the plane of layer-by-layer deposition. The basic tensile strength values for neat PLA are 36.5 MPa for the samples obtained by the FDM method and 72.6 MPa for the samples obtained by the injection moulding (Figure 8). This difference is due to the technique of producing the dumbbells for tests. Lower values of mechanical parameters of printed samples are mainly related to low inter-layer adhesion between the extrudate strands and the presence of air gaps between the applied layers. In the caseof injection moulding, the materials are more solid with little to no structural defects. The addition

of the SS-Limonene modifier increased the tensile strength of the printed samples, which brought them closer to the injection moulded ones. The highest value was observed for PLA with the content of 0.25% of the modifier, this value decreased with increasing concentration. This is mainly due to the improved flow of the polymer as shown in the capillary rheometry analysis of the samples (Section 3.1), as well as improved inter-layer adhesion (Section 2.4), which resulted in increased material consistency and improved fusing of the print paths (extrudate strands). For all the tested samples, high values of standard deviation were obtained, which is a characteristic feature of FDM printed objects due to the mentioned structural inconsistencies.

**Figure 8.** Tensile strength of SS-Limonene/PLA in 3D printing and injection moulding.

For the samples obtained by injection moulding, the tensile stress values are the highest for pure PLA and decrease with increasing loading of the modifier. This result can be explained by two reasons: one is the plasticizing effect of SS-Limonene, and the other is the presence of discontinuities in the polymer phase introduced together with the additive, that is the polymerized SS-Limonene agglomerates and additional air micropockets, as confirmed by the microscopy.

The elongation at maximum load for neat PLA samples obtained by 3D printing and injection moulding is characterized by similar values (2.43% and 2.29%, Figure 9). The addition of SS-Limonene to the samples obtained by the FDM method significantly improves the plasticity of the material. Higher elongation values in the case of modified samples indicate increased "mobility" of the polymer phase as a result of plasticization by SS-Limonene [35]. The plasticizer isolates the chains and spherulites of macromolecules, reducing the interaction between them. The highest value was obtained for PLA + 0.25% SS-Limonene, which was 4.21%. At higher loadings, the effect of the additive polymerization takes over and decreases the plasticizing effect. In the case of injection-moulded samples, the addition of the modifier gave negligible effects.

**Figure 9.** Elongation at maximum load of SS-Limonene/PLA in 3D printing and injection moulding.

#### 2.7.2. Bending Tests

The basic flexural parameters of the samples were determined. For the 3D-printed samples, the bending load was applied perpendicularly to the plane of layer-by-layer deposition. The basic values of flexural strength for pure PLA are 77.98 MPa for the samples obtained by the FDM method and 99.98 MPa for the injection-moulded samples (Figure 10). The flexural strength is also significantly lower for the neat PLA obtained by 3D printing—this is due to the presence of air gaps and limited inter-layer adhesion, the structural discontinuities acting as stress concentration points.

In the case of the 3D printing technique, the addition of the SS-Limonene modifier significantly increased the value of the flexural strength. The samples obtained by printing with the modifier showed similar mechanical properties to those mould-injected. The highest values were obtained for the systems containing 0.25% and 0.5% of SS-Limonene (respectively 97.61 MPa; 98.46 MPa). Higher concentrations of the modifier caused a slight decrease in the strength values in relation to the 0.25% and 0.5% systems, but they were still higher than for the neat PLA samples.

The samples obtained by injection moulding technique were characterized by high values of flexural strength ranging from 88.46 MPa to 100.30 MPa (5% SS-Limonene/PLA and 1% SS-Limonene/PLA, respectively).

The values of flexural modulus for both types of samples were basically unchanged regardless of the additive loading (Figure 11).

**Figure 10.** Flexural strength of SS-Limonene/PLA in 3D printing and injection moulding.

**Figure 11.** Flexural modulus of SS-Limonene/PLA in 3D printing and injection moulding.

#### 2.7.3. Impact Strength and Hardness

Impact strength of the obtained composite samples was determined. For the 3D-printed samples, the impact direction was perpendicular to the plane of layer-by-layer deposition. Impact resistance tests confirmed the beneficial effect of SS-Limonene (especially at lower loadings) on the tested samples, regardless of the processing technique used (Figure 12). The modifier, as in the case of the previously discussed mechanical tests, acts as a plasticizer, the brittleness of the polymer is reduced therefore, the obtained composite is able to absorb more energy during an impact. High standard deviations are characteristic of the measurement method. The downward tendency along with the increase in the modifier content indicates a limited dispersion of the modifier in the polymer matrix and compatibility of the system components. Hardness, in Shore D scale, was determined to be virtually unaffected by the additive and on the level of 82–84 for all the samples regardless of the SS-Limonene loading or the processing method.

**Figure 12.** Impact strength of SS-Limonene/PLA in 3D printing and injection moulding.

#### **3. Materials and Methods**
