**1. Introduction**

Three-dimensional (3D) printing is one of the most dynamically developing modern technologies. It was discovered and patented in the 1980s by Charles Hull (SLA technique) and was protected by a patent for 20 years [1]. The development and growth of interest in additive technologies has been going on continuously for about 15 years and is mainly caused by the expiration of patents, but also a decrease in printer prices and their increased availability. At that time, technological solutions became commercially available, initially only to the largest enterprises, now it is increasingly used by the sector of small and medium-sized enterprises, but also by individual consumers. Additive technologies are among those of much demand constituting the main pillar of Industry 4.0 [2]. Initially, 3D printing technology did not arouse much interest, and in the Gartner report from 2012 it was included in the area of the so-called 'Trough of Disillusionment' [3]. However, the aforementioned development of new techniques was so rapid that, in 2013, the position of the 3D printing sector changed dramatically and in 2015 Gartner published a separate report dedicated to it [4]. Additive technologies can be divided into: photochemical DLP [5], SLA [6,7], laser SLS [8], thermal FDM [9] or LOM [10].

One of the most popular additive techniques is FDM (fused deposition modelling). The technology originally developed by Stratasys involves extrusion from a die heated above the polymer melting point and then applying it layer by layer in the direction of the Z axis [11]. The extruded material is in the form of a filament with a diameter of usually 1.75 mm. The advantages of the FDM method are its versatility and accessibility [12], as well as the ease of designing and making a model of any shape and geometry [13]. The disadvantages include, first of all, insu fficient mechanical strength in the direction of the Z axis due to the appearance of air gaps between successively superimposed layers [14]. This e ffect does not occur in the case of other traditional methods, such as injection moulding, where solid objects are obtained.

Such air gaps can contribute to the appearance and initiation of cracks and material design defects. Therefore, attempts are continuously made to reduce poor quality of printed materials and improve the interfacial strength of printed models [15]. In addition, when compared to the traditional method such as injection moulding, 3D printing generates a number of technological problems related to insu fficient process speed [16] or product quality (often requiring additional post-processing) [17].

Significant disadvantages hampering the future of 3D printing also include high waste generation, higher than in traditional techniques such as extrusion or injection moulding. An attempt to eliminate unfavourable features by improving the processing device, which is the printer, have encountered significant limitations. In the FDM technique, several thermoplastics are usually used with the greatest emphasis on such polymers/copolymers as: PLA, PA, ABS, TPU. The same types of plastics are mostly used in 3D printing and mature processing techniques. FDM is micro-processing and has significant di fferences when compared to classic thermoplastics processing techniques, such as lower extruder pressure or smaller cross-sections of the canals in which the molten material flows. Therefore, plastics dedicated for 3D printing should be designed to show the properties addressing these di fferences (e.g., higher MFI/lower viscosity).

In the FDM technique, polylactide is the most commonly used polymer, mainly due to its ease of its processing, low thermal shrinkage [18] and biodegradability. Degradation rate is low enough to make it resistant to mild weather conditions [19] which is why, next to ABS, it is most often used in medicine [20]. The melting point of PLA is around 150–170 ◦C and is lower than those of many other popular polymers, it also requires much less energy due to low heat of fusion, so it can be widely used in various processing techniques [21]. One of the disadvantages of PLA limiting its application is low mechanical strength, especially the impact resistance [22]. In order to improve these parameters, structural fillers and plasticizers have been used, e.g., glass and carbon fibres, ceramic or metallic fillers, and glycols [23].

Chemical modifiers that improve the functional properties of composites, such as organic and organosilicon compounds, can also be applied. Limonene (4-isopropenyl-1-methylcyclohexene)—is the main component of oils obtained from waste citrus peels (biomass). It can be obtained by natural and synthetic methods, e.g., using pyrolytic processes [24]. Simple distillation or steam distillation of citrus peels makes it possible to recycle the waste citrus peels from food industry and, at the same time, to obtain pure limonene with a small amount of toxic waste. Annually, these methods produce over 70,000 tons of this compound. In 3D printing technology, it is used as a solvent for support materials made of high impact polystyrene [25]. From the point of view of synthetic applications, limonene should be classified as a green olefin that is subjectable to hydrosilylation reaction [26]. The use of limonene as a building block is one of the many necessary steps in creating a chain of products based on raw materials of natural origin with a lower environmental impact. In its pure form, however, it has a low boiling point for PLA processing (176 ◦C), which may cause its boiling during processing and, as a result, introducing gases into the polymer, as well as emission of vapours to the environment. Nevertheless, attempts have been made to use it in the processing of PLA, when it showed a plasticizing effect on the polymer matrix, which is a grea<sup>t</sup> advantage due to the brittleness of the neat polymer [27].

Polyhedral oligomeric silsesquioxanes are well-known organosilicon compounds, mostly recognized for their high symmetry, excellent solubility, or unparalleled simplicity of synthetic

protocols [28]. Among those, a subgroup called spherosilicates may be distinguished, sometimes identified as a di fferent group of compounds, and in such case, both spherosilicates and silsesquioxanes are collectively called cage siloxanes [29]. Due to their high thermal stability and good dispersion properties, they are considered interesting functional additives for polymer processing. In our previous works we have presented approaches towards processing of low-density polyethylene (LDPE) with polyhedral oligomeric silsesquioxanes [30], as well as spherosilicates [31]. These studies allowed determination of the critical level of practical loading in the LDPE matrix, which was much lower than what can be found in numerous literature reports, and was parallelly confirmed by Romo-Uribe et al. [32–34]. One derivative from among the tested ones was found to be particularly interesting, it was SS-Limonene, a product of limonene hydrosilylation with Octahydrospherosilicate. The results suggested its mildly plasticizing e ffect on the polymer matrix, besides its content improved thermal and mechanical properties of the obtained materials. Therefore, in this study, we decided to assess the applicability of this derivative as a functional and processing additive for PLA for 3D printing.

#### **2. Results and Discussion**

*2.1. Characterization of SS-Limonene (1,3,5,7,9,11,13,15-octa(Dimethyl((2-(4-methylcyclohex-3-en-1-yl)propyl) silyl)-Pentacyclo [9.5.1.13,9.15,15.17,13 ]Octasiloxane*

SS-Limonene (Figure 1) was prepared according to the synthesis procedure described in Section 3.3 and the reaction completion was determined by FT-IR spectra analysis, the disappearance of the characteristic signals assigned to stretching and bending vibrations of the Si-H group was observed at 2141 and 889 cm<sup>−</sup>1, respectively). Upon completion of the reaction, the hydrosilylation reached ~99% conversion. The structure and purity of the modifier was confirmed by NMR and MALDI-TOF-MS analyses.

**Figure 1.** Structure of SS-Limonene.

The purity and chemical structure of the synthesized compound was confirmed by NMR spectroscopy, with the following signals assignment:

1H-NMR (400 MHz, CDCl3): δ (ppm) = 5.36 (s, 8H, ring position 3), 2.00–1.87 (m, 24 H, ring positions 1, 2, 5), 1.78–1.56 (m, 24 H, ring positions 2, 5, =CH- isopropenyl methylidene), 1.63 (s, 24 H, -CH3 methyl attached to ring position 4), 1.34–1.18 (m, 16 H, ring position 6), 0.90 (d, 24 H, isopropenyl methyl), 0.75–0.69 (m, 8H, isopropenyl -CHaHb-), 0.52–0.44 (m, 8H, isopropenyl -CHaHb-), 0.15 (s, 48H, SiMe2); 13C-NMR (101 MHz, CDCl3): δ (ppm) = 133.96, 121.28, 121.26, 41.48, 41.33, 33.08, 32.93, 31.19, 31.12, 28.80, 28.24, 26.71, 25.71, 23.64, 23.63, 22.87, 22.48, 19.52, 19.22, 0.83, 0.72, 0.66; 29Si-NMR (79.5 MHz, CDCl3): δ (ppm) = 12.78 (SiMe2), −109.10 (core). IR (ATR): 2980–2867, 1252, 1169–1069, 869–734, 549. MALDI-TOF-MS: [M + Na]<sup>+</sup>: 2127.9581 (calc.), 2127.9606 (anal.)

#### *2.2. Density of SS-Limonene*/*PLA Blends and Mass*/*Quantity Waste Factor of the Printed Samples*

Densities of all samples were measured by the hydrostatic method. Measurements were performed for the samples of 1 cm in length. The average densities of all samples and the base sample of neat PLA were at the same level of around 1.24 g/cm3. The waste factor of printed samples was also analysed for the bars printed with 100% infill. Data are collected in Table 1. On the basis of the obtained data, the mass and the volume waste factors were determined according to the formula:

$$\mathcal{N}f[m,q] = \frac{\text{mass } (quantity) of \text{ correct samples} \times 100\%}{\text{mass } (quantity) of \text{ total samples}} \tag{1}$$


**Table 1.** Masses and the numbers of models examined in order to establish waste factor values.

The calculated waste factors clearly showed that with decreasing content of the SS-Limonene modifier, the amount of 3D printing waste decreases. The issues with printing the samples with high loading of SS-Limonene can be explained on the basis of the additive polymerizing and curing during the polymer processing, which is further explained in Sections 2.4 and 2.5.
