Barolo, Claudia

Bonomo, Matteo

Bracco, Pierangiola

Miravalle, Edoardo

Viada, Gabriele

Zanetti, Marco

English

Institutional Research Information System University of Turin

Attachment 2:RECYCLING OF NOVEL BIOBASED THERMOSET POLYURETHANE BASED ON DYNAMIC POLYMER NETWORKSEdoardo Miravalle1, Gabriele Viada1, Matteo Bonomo1,2, Claudia Barolo1,2,, Pierangiola Bracco1 and Marco Zanetti1,2,*1Department of Chemistry, NIS Interdepartmental Centre, University of Turin, via Pietro Giuria 7, 10125, Torino (Italy) 2 Instm Reference Centre, University of Turin, Via G. Quarello 15A, Turin, 10135, Italy * Corresponding authorINTRODUCTIONPolyurethanes (PU) are a versatile type of plastic that can be thermoplastic or thermosetting. They account for 8% of global plastic production, ranking them as the sixth most widely used polymer worldwide [1]. However, the extensive commercial use of PU results in significant waste generation, including end-of-life waste, post-consumer waste, and processing scrap. In Europe alone, customer-produced PU waste is estimated to range from 2.1 Mt to 3.6 Mt [2].  The treatment of PU waste is crucial, especially for thermoset PUs, as they cannot be directly industrially recycled, such as thermoplastic polymers. While current methods for recovering PUs often involve downgrade, new processes are being developed. One such process is based on a stress-relaxation behavior initially observed by Toblosky and Offenbach [3]. This behavior involves the exchange of covalent bonds within the polymer network, leading to rearrangement and the creation of a novel class of polymers called dynamic polymer networks (DPN). DPNs have the unique characteristic of maintaining crosslink density during bond exchange [4], which allows for the retention of material properties suitable for reworking purposes.  This study focuses on investigating the reprocessing capabilities of a newly synthesized biobased PU, exploring its potential within the context of DPNs.EXPERIMENTALMaterials: Sovermol® 750 (BASF) is solvent-free biobased polyether- and polyester- polyol made from soybean oil [5]. Tolonate™ X FLO 100 (Vencorex) is an aliphatic isocyanate polymer based on hexamethylene diisocyanate (HDI) containing 25% of renewable material derived from palm oil. [6]dibutyltin dilaurate (DBTL) from Sigma Aldrich (CAS:77-58-7)Characterizations: thermogravimetric analysis (TGA, TA instrument Q500) has been performed to assess thermal stability. The tests were conducted in a nitrogen atmosphere, ranging from 20°C to 600°C, with a heating rate of 10°C min-1. Differential scanning calorimetry (DSC, TA instrument Q200) was executed to determine the glass transition temperature. The samples (10-15 mg) were analysed under a nitrogen atmosphere, ranging from -90°C to 40°C, with a heating rate of 5°C min-1, before the heating and cooling an isotherm of 5 minutes were performed. Attenuated total reflection Fourier-transform infrared (ATR-FTIR, Perkin Elmer, Spectrum 100) spectra were recorded in the range of 4000–650 cm-1 at room temperature with a resolution of 4 cm-1 and 8 acquisitions.Mechanical properties were tested using a universal testing machine (Instron 3365) at a strain rate of 5 mm min-1 at room temperature, with dimensions of the specimens according to ASTM D628 type V. The average of five specimens obtained the reported values.  The reprocessing process was carried out by previously finely grinding the samples obtaining a homogeneous 0.25 mm powder, and then hot pressing it at 170°C for 45 minutes under 2 tons of pressure. The same conditions were used for each of the three cycles.RESULTS AND DISCUSSIONResults:The synthesis process involves mixing a trifunctional polyol with a catalyst and heating it, followed by degassing. This mixture is then combined with a difunctional isocyanate to obtain the final composition. After degassing again, the mixture is deposited into molds and cured to produce the material. The recycling process consists of two phases. In the first phase, the material is ground into a powder with particles no larger than 0.25 mm. In the second phase, the powdered material is placed in a mold and subjected to heat and pressure. This process is repeated multiple times to simulate recycling cycles. The resulting samples are homogeneous and compact. Tests indicate that the material is cross-linked and insoluble but swellable. Chemical structure analysis shows no changes, while thermal stability slightly decreases from 294°C to 275°C. However, this value remains consistent in subsequent processing stages. The glass transition temperature of the material remains around 28° C. The rigidity, measured by Young's modulus, remains nearly constant at 1.5 MPa. However, the tensile strength and elongation at break gradually increase with each subsequent processing cycle.Discussion:The synthesized PU is a cross-linked polymer and should, therefore, be non-fusible. However, the collected data demonstrate that the polymer can be reconsolidated after grinding. IR spectra indicate no chemical variation in the structure, confirming that no different bonds were formed compared to those originally present in the material, and the overall molecular architecture corresponds to that of a thermoset, rendering it insoluble.  This behavior can be explained by the theory of dynamic polymer networks (DPN), which are a type of thermosetting polymer material that incorporates dynamic covalent chemistry into the polymer network. When a stimulus (such as heat, pressure, etc.) is applied to the material, these dynamic bonds become active and can be broken or exchanged with other functional groups, allowing the polymer network to change its topology. Despite the Young's modulus remaining unchanged, the increase in elongation at break and tensile strength with an increasing number of reprocessing cycles suggests an increase in the degree of cross-linking. However, this aspect should be further investigated through measurements of the degree of cross-linking. CONCLUSION In this preliminary study, we have demonstrated that it is possible to produce thermosetting polyurethanes that can be easily recycled at their end-of-life using simple methods that do not require complex processes reserved for thermosetting polymers, such as chemical recycling.References[1]  “Plastics - the Facts 2019,” pp. 14, 35, 2019.[2]  J. Datta, J Polym Environ, vol. 26, no. 1, pp. 166–174, 2018, doi.org/10.1007/s10924-016-0932-y[3] J. A. Offenbach and A. v Tobolsky, J Colloid Sci, vol. 11, pp. 39–47, 1956, doi.org/10.1016/0095-8522(56)90017-4[4] J. Zheng et al.,ok, attendo il suo contatto, a presto Materials Today, vol. 51, pp. 586–625, 2021, doi.org/10.1016/j.mattod.2021.07.003 [5] BIO-BASED PLANT OIL POLYMERS AND COMPOSITES; Madbouly, S.A., Zhang, C., Kessler, M.R., Eds.; Elsevier Inc., 2016; ISBN 9789896540821.[6] 1. Niesiobędzka, J.; Datta, J. Challenges and recent advances in bio-based isocyanate production. Green Chem. 2023, 2482–2504, doi:10.1039/d2gc04644j.

Recycling Of Novel Biobased Thermoset Polyurethane Based On Dynamic Polymer Networks

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Recycling Of Novel Biobased Thermoset Polyurethane Based On Dynamic Polymer Networks

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