Fully Recyclable Cured Polymers for Sustainable 3D Printing

The most prevalent materials used in the Additive Manufacturing era are polymers and plastics. Unfortunately, these materials are recognized for their negative environmental impact as they are primarily nonrecyclable, resulting in environmental pollution. In recent years, a new sustainable alternative to these materials has been emerging: Reversible Covalent Bond‐Containing Polymers (RCBPs). These materials can be recycled, reprocessed, and reused multiple times without losing their properties. Nonetheless, they have two significant drawbacks when used in 3D printing. First, some require adding new materials every reprinting cycle, and second, others require high temperatures for (re)printing, limiting recyclability, and increasing energy consumption. This study, thus, introduces fully recyclable RCBPs as a sustainable approach for radiation‐based printing technologies. This approach enables multiple (re)printing cycles at low temperatures (50 °C lower than the lowest reported) without adding new materials. It involves purposefully synthesized polymers that undergo reversible photopolymerization, composed of a tin‐based catalyst. An everyday microwave oven quickly depolymerized these polymers, obtaining complete reversibility.


Introduction
Additive manufacturing, commonly known as 3D printing, has gained significant attention for fabricating complex structures using polymers.While polymers offer advantages such as high volume-to-weight and strength-to-weight ratios, existing recycling methods often result in degraded properties, highlighting the need for better sustainable solutions. [1,2]Recent approaches for addressing the recycling of plastics involve Reversible Covalent Bonds Containing Polymers (RCBPs), also known as DOI: 10.1002/adma.202307297   Covalent Adaptable Networks (CANs) or vitrimers.[3] In general, RCBPs are divided into two types of bonds: dynamic bonds, also known as associative CANs, and general bonds, also known as dissociative CANs. [1]Dynamic bonds, like the thermal-triggered transesterification of -hydroxy esters, react to a similar factor to form or break the bonds.Thus, the cross-linking density remains constant, and reprocessing is possible due to chain slipping.General bonds, on the other hand, form under specific stimuli and deform under different ones.An example of such bonds is the [4 + 2] cycloaddition Diels-Alder (DA) reaction, which forms cycloadducts at low temperatures and deforms them at higher temperatures.
Current approaches for RCBPs' 3D printing, however, have significant drawbacks: they either, rely on the addition of new curable materials like acrylates every reprinting cycle as none of the dynamic bonds include photopolymerizable double bonds as was demonstrated by Bowman et al. [4,5] and Li et al., [6] or require high temperatures for a long time, sometimes even hours, during (re) printing (120-200 °C). [1,3,7]This limits the number of recycling cycles due to degradation of the printed polymer or due to irreversible side reactions, as well as high energy consumption during the recycling and reprinting processes.
In addition to typical RCBPs' 3D printing methods, reversible addition-fragmentation chain-transfer (RAFT) polymerization has also been used in 3D printing. [8,9]RAFT is a common method of reversible deactivation radical polymerization (RDRP), which also includes stable radical-mediated polymerization (SRMP) and atom-transfer radical polymerization (ATRP).Whereas RAFT polymerization is considered a reversible process, it is not categorically fully reversible.The process is characterized by a dynamic equilibrium between dormant and active species, and the rate of activation and deactivation of the dormant species determines the reversibility of the process.In such processes, usually the reversibility of the polymerized materials is limited, and the original monomers are not obtained. [10]A recent study by Anastasaki et al. has shown that it is possible to reverse RAFT polymerization and regenerate the monomer through a catalystfree depolymerization approach, under specific conditions. [11]Although promising, it has yet to be applied in applications like 3D printing.Moreover, this method still requires the addition of new photoinitiators every reprinting cycle.
This study (Figure 1) presents fully recyclable and (re)printable polymeric compositions to address the above limitations.3D printing is demonstrated for direct ink writing (DIW) process combined with reversible photopolymerization, eliminating the need for high temperatures processing or adding new materials to achieve (re)printability.This is obtained by introducing new monomers that undergo [2 + 2] and [4 + 4] cycloaddition reactions.These reversible reactions occur only under light irradiation, forming new products (cycloadducts) with new  and  bonds [12][13][14][15] (Figure 2; Figure S1, Supporting Information).Moreover, the printing compositions include our recently reported tinbased tetradentate catalyst, which is capable of rapidly performing photopolymerization reactions. [16]A day-to-day microwave oven proved to be an efficient method to and fast de-crosslinking to obtain monomers enabling the printed polymer's recyclability for reprinting without further processing.

Synthesis and Printing
First, a trifunctional monomer, TETA-CA (Figure 2; Figures S2-S4, Supporting Information), was synthesized by reacting triethylenetetramine (TETA) with cinnamaldehyde (CA), a known component that can undergo cycloaddition reactions, [17,18] yielding a material that can be printed via these reactions.The reaction conditions were selected based on previous studies conducted by our group. [16]Additional details regarding the synthesis can be found in the Experimental Section and the Supporting Information.
Printing was performed using a DIW printer equipped with a 365 nm UV LED array (3.2 mW cm −1 ), enabling photopolymerization by [4 + 4] and [2 + 2] cycloaddition reactions.Because the original viscosity was too high for printing even at high temperatures due to inter-and intramolecular bonds, a solvent, benzyl alcohol (BnOH), was added to reduce the viscosity.16 wt% was found as the lowest concentration that still enabled printing via DIW.BnOH itself has been selected as it has a relatively low volatility and is nonreactive in the current composition.Therefore, the amount remains constant, as verified by thermogravimetric analysis (TGA) measurements, thus eliminating the need for solvent replenishing after every printing cycle.Continuous UV exposure during printing ensured sufficient conversion and shape retention of the extruded material.The printing conditions were optimized by assessing the viscosity-temperature relationship (Figure 3A).Below 60 °C, the viscosity was too high for extrusion, even with the solvent, while above 70 °C, the viscosity became too low, causing shape deformation due to rapid polymer flow.
Ensuring that printed structures maintain their desired shape is crucial to avoid distortion caused by overly rapid material flow before curing.A parallel plate rheometer recovery test was conducted at 70 °C (Figure 3B) to evaluate the shape retention of printed materials.The results demonstrated high viscosity recovery and, thus, high shape retention, assumingly due to the formation of inter and intramolecular bonds.The viscosity was measured throughout the test by adjusting the frequency from 0.1 to 100 Hz and then back to 0.1 Hz, which replicated the pre-, extrusion, and postextrusion phases.To demonstrate the effectiveness of the printing process, printing at 70 °C was performed using different STL files, including a hollow object with overhanging parts, which is challenging in DIW printing (an igloo, Figure 4A).The printed sample was postcured under 365-405 nm lamp (4.7 mW cm −1 ) for one hour, achieving a conversion of 81.4 ± 0.5% (Figure 5A).It should be noted that this printing temperature was 50 °C lower than the previously reported lowest temperature for RCBPs printing based on the Diels-Alder reaction. [1,19]Moreover, a TGA test showed that no solvent's evaporation occurred during the printing process (Figure 2D).

Recycling
Typically, reversing cycloaddition reactions requires irradiation at the harmful range of UVC (<260 nm for cinnamaldehydebased moieties [17,18] ).The instability of four and eight-membered rings, resulting from bond angle stress, makes them susceptible to ring-opening via excitation under UVC.An alternative  approach was explored to avoid UVC irradiation, based on microwave irradiation (25-38 mm), which is known to cause rapid heating through the rotation of polar molecules (C-N bonds in the present molecule, TETA-CA-based polymer).Some cycloaddition reactions, such as the [4 + 4] of anthracenes, [20][21][22] can dissociate under high temperatures that cause vibrations of the rings, leading to steric stresses, which results in conversion to the more steric stable form, which are the original molecules.Following a study by Beves et al., [23] it was hypothesized that microwave irradiation can cause a similar effect at much lower temperatures, as it effectively initiated the adducts' vibrations.This may explain why simple heating at 180 °C, as will be discussed later, did not cause the cycloreversion, which may occur at much higher temperature that should be avoided to eliminate potential irreversible reactions.Thus, postcured printed samples were irradiated in a kitchen-type microwave oven (216 W) for up to 15 min.The recyclability was evaluated by monitoring the double bond formation.Thus, changes in absorbance and C-H double bonds' shifts were followed in the 1 H-NMR spectra (around ≈6.6 ppm), normalized to the aromatic C-H bonds (≈7.3 ppm), and compared with precured monomers.As shown in Figure 4B, 97.6 ± 0.3% of the double bonds were reformed after 10 min of irradiation (labelled as "recycling conversion").Interestingly, a subsequent decrease in the double bonds was observed after 10 min (84.9 ± 1.3% at 15 min) that may be attributed to either long-term microwaveinduced cycloaddition [24][25][26] or an irreversible reaction involving the double bonds, as supported by changes in the 1 H-NMR signals of HC═CH (Figure S9, Supporting Information).
As a result of the microwave irradiation, the solid 3D printed object loses its structure during the irradiation, reaching full liquification after 10 min (Figure 4B,C), ready for re-printing.It should be noted that microwave irradiation caused an increase in the temperature of the printed object to 180 °C, as revealed by thermal imaging (Figure S7, Supporting Information), followed by liquification.However, heating the samples for 20 min at 180 °C did not cause a visible change in the objects.Therefore, it can be concluded that the cycloreversion of the polymer is not a simple thermal reaction.Moreover, a stress relaxation test of a postcured sample compared to a recycled one (Figure S9F, Supporting Information) also showed a significant difference between simple heating and microwave irradiation; The heating caused a small change in shear modulus and reached a constant value of ≈17 MPa after 1100 s, compared to the microwave treated sample that reaches a zero stress at the same time.
Eleven printing-recycling cycles were conducted to assess the re-processability of the printing composition.As presented in Figure 5A, the double bonds' content of the first and 10 th recycling cycles (Figure S9, Supporting Information) is 97.6 ± 0.3% and 97.2 ± 0.1%, respectively, indicating the excellent recyclability of the proposed innovative system as almost all the double bonds were reformed.
The gel content of the 11 th postcured printed samples was assessed using a swelling test in ethanol.The samples were analyzed both before printing (10 th recycling) and after postcuring (11 th printing cycle).Figure S5 (Supporting Information) displays the results, which reveal that before printing, and thus after recycling in a microwave oven, the materials completely dissolved in the ethanol, with a gel content of 0.00 ± 0.01%.However, following printing and postcuring, the gel content significantly increased to 95.9 ± 0.5%, providing clear evidence of the effectiveness of the recycling process.
Further analysis was performed to compare the mechanical and thermal properties of the first printed sample and the 11 th one (Figure 5B,C).Dynamic mechanical analysis (DMA) tests using a dual cantilever bending mode showed nearly identical glass transition temperatures (T g ) of ≈35 °C (Figure S8, Supporting Information) for the two samples.DMA test of a dried sample was also conducted, using the 11 th printed sample.Due to the brittleness of the sample at the low temperature, a single-cantilever mode was used, instead of the dual one.As shown in Figure S8 (Supporting Information), the measured T g of the dried sample was slightly higher, 48 °C, compared to 35 °C of the wet sample.The storage and loss moduli of the wet samples at 25 °C were also similar, with almost negligible higher values observed for the 11 th printed sample (E′ = 645 MPa and E′′′′ = 124 MPa) compared to the first one (E′ = 637 MPa and E′′′′ = 123 MPa).
Additionally, tensile tests revealed similar behavior of the printed samples (Figure 5B).The as-printed samples show high elongation values and low Young's moduli, 1372 ± 362%, 1425 ± 302%, and 34.0 ± 3.4 MPa, 35.5 ± 4.6 MPa for the 11 th and the 1 st printed cycles, respectively.These samples contain 16% BnOH, and their properties are comparable to commercial printing elastomers like NinjaFlex polyurethane (600% elongation and Young's modulus of 12 MPa) and Stratasys' FDM TPU 92A (552% elongation and Young's modulus of 15.3 MPa).Dried samples, after evaporation of the BnOH, were also tested (Figure 5C) and found to be less ductile but with higher Young's moduli and stress at yield: elongation of 920 ± 59% and 928 ± 49%, and Young's moduli values of 1.12 ± 0.20 and 1.06 ± 0.15 GPa for the 11 th and 1 st cycles, respectively.The mechanical properties of the printed objects are within the range of commonly used elastomers and polymers, as presented in Figure 5D, which summarizes the properties of 661 3D printable commercial materials.

Conclusion
In conclusion, this research presents a sustainable approach to 3D radiation printing by utilizing reversible cycloaddition reactions.The new synthesized monomer (TETA-CA) demonstrates showed printability at significantly lower temperatures than previously reported RCBPs.The presented approach allows multiple printing cycles without compromising the printed objects' mechanical and thermal properties and without replenishing materials, which are necessary in most current solutions involves light-induced RCBPs' 3D printing.This breakthrough will facilitate using sustainable raw materials, ultimately contributing to a more efficient and eco-friendly perspectives for 3D printing.This study's findings offer promising implications for advancing sustainability, polymers, and material science.
Structural Analysis: To analyze materials chemical compositions, IR spectroscopy, 1 H-NMR, and gas chromatography-mass spectroscopy (GC-MS) were used.IR was recorded using the ATR-IR method, on a Bruker Alpha-P machine (Brucker, USA), in the range of 400-4000 cm −1 . 1 H-NMR was tested using DMSO-d6 as solvents and was performed in a 400 MHz spectrometer (Ascend 400 Neo by Brucker, USA) with tetramethylsilane (TMS) as an internal reference.GC-MS was conducted using Agilent 8860 GC coupled to an Agilent 5977B MSD equipped with an Agilent HP-5MS UI capillary column (30 m × 0.25 mm × 0.25 μm).GC-MS conditions were as follows: Helium as a gas carrier, 1.0 mL min −1 Column flow, inlet temperature: 280 °C, inlet mode: split 50:1, interface temperature: 300 °C, ion source temperature: 320 °C, spectral range of the electron ionization: 40-250 amu.
Materials' absorbance spectra were recorded using a UV-vis spectrophotometer (UV-1900i, Shimadzu, Japan) between 800 and 200 nm using EtOH solutions of 2.25 × 10 −8 [g mL −1 ] in 1 cm path length's quartz cuvettes.The printed and postcured samples were tested using a thin film (0.05 mm) on a poly(ethylene terephthalate) sheet (75 μm).All spectra were normalized to the internal C-C absorbance at 217 nm, following by the measurements of the normalized signals' changes.
Swelling tests of TETA-CA samples were conducted according to ASTM D2765.The samples were swollen in ethanol at 35 °C for 24 h.Later, the solvent was removed from the samples using a Büchner funnel and cellulose filter paper.The swollen samples were then dried in a vacuum oven at 25 °C until reaching constant weight (≈24 h).To eliminate the possible effects of BnOH on the weight changes, as it is also soluble in ethanol, the tested samples were initially dried at 55 °C under vacuum for 12 h until their weight remained constant.
Thermal Analysis: Dynamic mechanical analysis (DMA) of printed samples was conducted by TA Instruments' DMA Q800 V21.3 Build 96, TA Instruments, USA, from 0 to 80 °C at a rate of 3 °C min −1 in a dual cantilever bending mode with an amplitude displacement of 8 μm and a frequency of 1 Hz.The DMA's samples were printed following the standard ISO 6721.Dried sample of the 11 th printing cycle were tested using the same instrument, though in a single cantilever bending mode, from 0 to 90 °C at a rate of 3 °C min −1 with an amplitude displacement of 15 μm and a frequency of 1 Hz.The drying process took place for 24 h (until the samples' weight remained constant) under vacuum at 25 °C.
To understand the pot-life of TETA-CA with the catalyst at 70 °C, an isothermal differential scanning calorimetry (DSC) was also conducted for 240 min using TA Instruments' DSC Q200 V24.11 Build 124, TA Instruments, USA.A heat-cool-heat cycle of TETA-CA with Sn(PA-MPIB) was also conducted from −25 to 200 °C at 10 °C min −1 heating rate.Samples' temperature dissipation was measured using a thermal camera (FLIR-E63900, FLIR, Sweden).Thermogravimetric analysis (TGA∖DSC Strat System 1, Mettler Toledo, Switzerland) analysis of a postcured printed TETA-CA sample was conducted between 50 and 300 °C using a heating rate of 10 °C min −1 .
Rheological Analysis: A parallel-plates rheometer (Discovery HR-1, T.A. Instruments, USA) was used to characterize the rheology of TETA-CA.To test the temperature effect on the monomer's viscosity before curing, a temperature ramp mode was used with heating rate of 2.5 °C min −1 from 25 to 150 °C at 100 Hz.The recovery of the monomer at 70 °C was conducted at three non-stop consecutive steps at 0.1, 100, and 0.1 Hz.
Stress relaxation of postcured and recycled samples of TETA-CA was conducted using the same parallel-plates instrument for 1200 s using 5% strain.All these samples contained 16 wt% solvent (BnOH).
Mechanical Analysis: The samples' tensile tests were conducted following ASTM D638-Type IV standard using a mechanical tester (INSTRON 4481, Instron, USA).All samples were dried before testing under vacuum for 24 h at 25 °C until the samples' weight remained constant.
Printing: Printing of TETA-CA was conducted using Hyrel System 30M with a 365 nm UV array (3.2 mW cm −1 ).All models of the STL files' Gcodes were generated by Slic3r.The material was printed at 60, 70, 80, and 90 °C using the built-in heater of the KR2 Extruder head with 0.5 mm gauge nozzle.The stage itself was kept under room temperature.The printing parameters were as follows: Layer thickness (mm): 0.4, Infill density: 100%, Perimeter speed (mm s −1 ): 5, Infill speed (mm s −1 ): 5, Travel speed (mm s −1 ): 60.Based on these parameters, the shear rate during the printing was ≈102 s −1 .
During the printing process, the sample was continuously irradiated at 100% intensity.Due to the low-intensity lamp, each layer was irradiated only for a few seconds.Thus, a postcuring was required, which was conducted using an Asiga Flash Post Curing Unit (365-405 nm, 4.7 mW cm −1 ) for one hour.The samples' conversion after printing and after postcuring was conducted following the changes in the UV absorbance around 258 nm, following previous studies. [16]ecyclability: To achieve the recycling of the cured polymer, TETA-CA's samples were heated in a microwave oven for different periods using 216 W intensity.To overcome overheating and degradation, every 30 s, the oven stopped for 15 s.The heating was carried out using Sauter's MW2031W microwave (Sauter, China).
Conversion of the recycled materials was calculated in comparison to preprint samples, following both the changes in the absorbance as discussed above, or by following the in the C-H double bonds (≈6.6 ppm) normalized to the aromatic C-H bonds (≈7.3 ppm) (Equation 1).It was found that 2.4± 0.4% conversion remained after the first microwave recycling and 2.8± 0.1% after the 10 th .Compared to the 10min cycle, 15-min irradiation under the same conditions caused a much higher double bonds' conversion, as around 15% conversion was calculated.

Figure 1 .
Figure 1.A schematic representation of this work's idea: printing the monomers into polymers using UV light, then recycling them into monomers using a microwave oven, and then reprinting them.Both the 1 st printing cycle and 11 th were printed under the same conditions (70 °C, 365 nm continuous irradiation), showing no significant differences in the resolution.All scale bars are of 1 cm.

Figure 2 .
Figure 2. Synthesis and cross-linking illustrations of the functional monomer.A) A schematic illustration of TETA-CA.B) Different possible cycloaddition reactions of TETA-CA.C) A schematic illustration of the cross-linked polymer.Sn(PA-MPIB) is the catalyst and BnOH stands for benzyl alcohol, which serves as the solvent.

Figure 3 .
Figure 3. TETA-CA's printing conditions.A) Complex viscosity of TETA-CA with the catalyst Sn(PA-MPIB) (20 mol % of CA) as a function of temperature.B) A recovery test of the formulation at 70 °C.C) Different samples printed by DIW from 60 to 90 °C.All scale bars are 1 cm.D) TGA results of TETA-CA after printing and postcuring.

Figure 4 .
Figure 4. Recycling the polymer in a microwave oven.A) A printed sample with overhanging parts.The black scale bar represents 1 cm, whereas the pale blue represents 0.5 mm.The figure shows the STL file, the printed layer-by-layer structure, and magnification of a section of the object emphasizing the layer structure and dimensions.B) Conversion of the adducts' dissociation (recycling conversion, percentage of the recovered double bonds) as a function of microwave irradiation time.The pale blue shade represents the error bars.The conversion was calculated based on the changes in the samples' absorbance and fluorescence.C) pictures of postcured samples after different irradiation time intervals in a microwave oven (216 W).

Figure 5 .
Figure 5. Mechanical Analysis and reprocessability.A) Polymer's curing conversion (percentage of the cycloadducts' formation) based on 1 H-NMR and UV-vis.The similar conversion between the 1 st and 10 th recycling demonstrates the stability of the process.B,C) Tensile tests of samples with 16 wt% solvent (B) and after drying (C), following ASTM-D638 type IV standard.The error bars, based on five samples, are represented as shades: red and blue for the 1 st and 11 th printing cycles respectively.D) A comparison of TETA-CA properties and commercial printable materials.The graph was created using the Ansys GRANTA EduPack software, ANSYS, Inc., Cambridge, UK, 2023 (www.ansys.com/materials)'sdatabase.The materials for comparison were chosen by selecting "additive manufacturing" in the ProcessUniverse database.PF -Phenol formaldehyde.PE-HD -High Density Polyethylene, Silicone, VMQ -Polydimethylsiloxane/Vinyl Methyl Silicone, PA -polyamide.