Thermoset Polyurethane Research Articles

Low-temperature chemical upcycling of poly(ethylene terephthalate) waste to recyclable polyurethane thermosets using biomass-derived materials

Addressing the global challenge of plastic waste, particularly poly(ethylene terephthalate) (PET), requires innovative chemical upcycling strategies. In this study, we report a sustainable approach for transforming PET waste into chemically recyclable polyurethane (PU) thermosets. Using a low-temperature glycolysis process with potassium carbonate, 1,4-butanediol and chlorobenzene, bis(4-hydroxybutyl) terephthalate (BHBT) was obtained with a conversion rate exceeding 99 % and an isolated yield of 55.4 %. The BHBT was subsequently used to synthesize PU thermosets with bio-based pentamethylene diisocyanate trimer and 2-methyltetrahydrofuran as a solvent. The resulting thermosets demonstrated excellent mechanical properties, including a tensile strength of 21.8 MPa, elongation at break of 151 %, and a Young’s modulus of 279 MPa. The PU thermosets were chemically recyclable, enabling a closed-loop process where BHBT was recovered with 84.5 % yield via glycolysis. Furthermore, the BHBT-based PU thermosets were successfully applied to carbon fiber-reinforced polymer composites, maintaining their mechanical integrity after recycling. This work highlights a promising route for the upcycling of PET waste, contributing to the development of sustainable polymer materials and supporting the principles of a circular economy.

Curing kinetics of block copolymerization thermoset polyurethane by non‐isothermal DSC method

AbstractPolyurethanes consist of hard and soft segments, with the hard segments typically comprising blocks containing isocyanate groups, while the soft segments often consist of chains terminated with hydroxyl groups. The hard segments contribute to the mechanical properties of polyurethanes, while the soft segments provide elasticity and flexibility. The essence of curing lies in the mixing of hard and soft segments and subjecting the system to high temperatures. The curing kinetics of PEG/PCL/IPDI/N100 in‐situ block copolymers were investigated using dynamic isothermal differential scanning calorimetry (DSC). Key curing parameters were obtained from non‐isothermal DSC curves, and these parameters were incorporated into an n‐order reaction model to determine the reaction type and apparent activation energy, thus deriving the reaction equation. The curing kinetics of PEG/PCL/IPDI/N100 in‐situ block copolymers follows an n‐th order autocatalytic reaction, where n is 1, and the activation energy is approximately 52.79 kJ/mol. Based on the reaction equation, the curing process conditions, namely the initial curing temperature (Ti), the peak exothermic temperature (Tp), and the finishing temperature (Tf), were deduced. Studying the curing kinetics of thermosetting polyurethanes can deepen our understanding of the interactions and reaction mechanisms between reactants under different conditions, thereby enhancing our grasp of the curing process. Furthermore, research into curing kinetics can provide a basis for optimizing preparation processes.

Robust Self-Healing Adhesives Based on Dynamic Urethane Exchange Reactions.

Thermoset polyurethanes (PUs) have been successfully reprocessed as covalent adaptable networks (CANs) by catalyzing carbamate exchange. Here we extend bond exchange beyond the internal network cross-links to create a dynamic urethane adhesive. Interfacing PU CANs to substrates with nucleophilic functional groups creates adhesives capable of reversible transcarbamoylation with the substrate, which has not been demonstrated previously by CAN adhesives. Two types of thermoset PU films were synthesized, one containing the green carbamate exchange catalyst Zr(tmhd)4 and the other containing no catalyst. Although otherwise identical in chemical and network properties, as indicated by FT-IR spectroscopy and dynamic mechanical thermal analysis (DMTA), the film containing catalyst showed dynamic bond exchange behavior through stress relaxation analysis. When evaluated as an adhesive, the CAN film exhibited self-healing properties and retained its adhesive strength for five cycles, which is attributed to reversible covalent bonding to the glass substrate. This work expands industrially relevant CANs to structural adhesives and demonstrates their potential value in an application that presently employs PUs as single-use materials.

Functionalization and Repurposing of Polypropylene to a Thermoset Polyurethane.

Developing effective recycling pathways for polyolefin waste, enabling a move to a circular economy, is an imperative that must be met. Postuse modification has shown promising results in upcycling polyolefins, removing the limitation of inertness, and improving the final physical properties of the recycled material while extending its useful lifetime. Grafting of maleic anhydride groups to polypropylene is an established industrial process that enhances its reactivity and provides a convenient route to further functionalization and upcycling. In this work, maleic anhydride grafted polypropylene was hydroxylated and subsequently cured with a diisocyanate to form a thermoset polyurethane (PU). The crystal structure (unit cell and lamellar structure) of the polypropylene (PP) was preserved in the PU. At room temperature, the PU showed a high modulus due to the crystallization behavior of the PP; upon increasing the temperature above the melting temperature, the modulus decreased to a rubbery plateau, consistent with formation of a network. The resulting PU showed a higher glass transition temperature and lower degree of crystallinity than its PP predecessor due to the crosslinked nature of the polymer. The mechanical integrity of the PU was maintained through several reprocessing cycles due to the melt processability enabled by the presence of a urethane exchange catalyst. This functionalization and upcycling route thus offers a promising alternative to repurposing PP waste in which the creation of melt-processable thermoset polymers expands applications for the materials.

The Multifaceted Role of Water as an Accelerator for the Preparation of Isocyanate-Free Polyurethane Thermosets

The Multifaceted Role of Water as an Accelerator for the Preparation of Isocyanate-Free Polyurethane Thermosets

Recycling of Commercially Available Biobased Thermoset Polyurethane Using Covalent Adaptable Network Mechanisms.

Until recently, recycling thermoset polyurethanes (PUs) was limited to degrading methods. The development of covalent adaptable networks (CANs), to which PUs can be assigned, has opened novel possibilities for actual recycling. Most efforts in this area have been directed toward inventing new materials that can benefit from CAN theory; presently, little or nothing has been applied to industrially producible materials. In this study, both an industrially available polyol (Sovermol780®) and isocyanate (Tolonate X FLO 100®) with percentages of bioderived components were employed, resulting in a potentially scalable and industrially producible material. The resultant network could be reworked up to three times, maintaining the crosslinked structure without significantly changing the thermal properties. Improvements in mechanical parameters were observed when comparing the pristine material to the material exposed to three rework processes, with gains of roughly 50% in elongation at break and 20% in tensile strength despite a 25% decrease in Young's modulus and crosslink density. Thus, it was demonstrated that theory may be profitably applied even to materials that are not designed including additional bonds but instead rely just on the dynamic urethane bond that is naturally present in the network.

Shape‐memory thermosets: Tailoring the structure for toughness improvement and scratch healability

AbstractA straightforward but effective approach to fabricate shape‐memory polyurethane thermosets by tailoring the soft phase composition of two‐ and four‐armed polycaprolactone (PCL) polyols is described. As the crystallization of the PCL soft phase has an effect on driving the self‐assembly of H‐bonds, a good balance of tensile performance and thermo‐healability can be achieved by regulating the ratio between PCL‐diol and PCL‐tetrol in the soft phase. The inferior ability of crystallization of the segments near the branching point gives rise to enhancement of the both overall healability and toughness of the network. Healings of punctured holes and cuts are assessed using optical microscopy (OM), field‐emission scanning electron microscopy (FE‐SEM), tensile analysis, and two‐electrode impedence characterization. The obtained network with the best healing behavior has a soft phase composition with the PCL‐diol/PCL‐tetrol molar ratio of 2/1, a tensile toughness of 64 MPa J−1 and a healing efficiency (estimated by recovery of tensile strength) of nearly 80%.Highlights The composition of the soft phase of polyurethane networks was tailored. Two‐ and four‐armed polycaprolactones were used for the network structure. A good balance between mechanical and healing performance was obtained. Healing of complete cuts showed tensile strength recovery of nearly 80%.

Programming degradation and closed-loop recycling of ultra-stable bio-based thermosetting polyurethane relied on double-locked covalent adaptable networks

Castor oil (CO) derived thermoset polyurethanes are usually limited by their poor mechanical properties and the difficulty in removing or recycling because of the aliphatic or alicyclic backbone and their permanent cross-linking structures. Although covalent adaptable networks (CANs) based on dynamic bonds have been proposed, it is still highly desired to fabricate polymer CANs with high performance, stimulus resistance, stepwise degradation and closed-loop recycling from renewable CO for sustainable development. In this study, CO-based poly(urethane urea) (PU) thermosets relied on double-locked covalent adaptable networks (DLCANs) were proposed to achieve programmable degradation and closed-loop recycling. From a CO-derived isocyanate monomers and two dynamic curing agents of aminophenyl disulfide (AFDS) and aminophenyl diimine (AFDI), bio-based thermosets of PU DLCANs were prepared. The bio-based PU DLCANs were ultra-stable and still kept cross-linked upon disadvantageous chemical stimuli instead of dissociating like the conventional CANs. Moreover, the bio-based PU DLCANs became degradable only by applying programmed stimuli, when the two types of dynamic cross-links in parallel connection were both broken step by step. Meanwhile, the broken cross-links could be reformed when dynamic bonds regenerated upon heating, leading to the closed-loop recycling of the bio-based PU DLCANs. Besides, being applied as coatings, the bio-based PU DLCANs exhibited UV shielding, self-healing properties and long-term stability as well as programming removable properties. The bio-based thermosets relied on DLCANs have the potential to guarantee the reliability of polymeric CANs and maintain the closed-loop recycling by applying programmable stimuli, which is quite meaningful and practical for life-time environmental-friendly coatings.

Recyclable azine polyurethane thermosets and carbon fiber reinforced composites with excellent thermostability and mechanical properties

Carbon fiber reinforced composites (CFRCs) have received increasingly concerns and widely used in high-end applications, owing to their outstanding mechanical properties. Most of the polymers used in CFRCs were thermosets. However, a large number of applications brought enormous wastes due to the stable structure of thermosets which is difficult to degrade or recycle. CFRCs based on dynamic covalent polymers (DCPs) were developed to resolve the recycling problem. Polyurethanes were used for fabricating CFRCs applied in energy absorbing materials, vehicle interiors, sporting goods, and constructing materials, owing to excellent toughness, bonding abilities and low VOCs. Although some recyclable PU CFRCs were developed to resolve the recycling of CFs, there remain the problems of excess catalysts, low chemical resistance or thermostability due to the dynamic feature of DCPs. Herein, a series of azine polyurethane thermosets (APUTs) were prepared from hexamethylene diisocyanate (HDI) trimer and dihydroxy azines with different lengths of carbon chains. The resulting APUTs displayed excellent thermal stabilities with all Td, 5 % exceeding 320 °C, owing to high exchange temperature of azine moieties and high crosslinking density. APUTs-6 showed the highest tensile strength of 36.72 ± 1.18 MPa. APUTs-6 could be easily degraded in 0.1 M acetone/water solution at 50 °C. APUTs-6 showed excellent reprocessing properties and tensile strength could maintain 83.1 % of its initial one. The APUTs-6/CFs composites exhibited excellent mechanical properties, chemical resistance and recyclability. CFs could be readily recycled by acid degradation and maintain excellent mechanical properties. The tensile strength of recycled composites could recover 94.36 % of the original one after two times of recycling. The recovery ratio of tensile strength of recycled CF filaments was 95.7 %. The application of phenyl azine provided a feasible solution for the industrialization of recyclable CFRCs.

Controlling associative transcarbamoylation reactions by light in dynamic polyurethane networks using reversible spiropyran photoswitches

Polyurethanes represent a versatile class of polymers and are one of the most employed in the thermoset market. However, due to their thermodynamically stable carbamate bond, they suffer from a lack of reprocessability, recyclability, and degradability, and are therefore usually discarded after use. Recently, transcarbamoylation in polyurethane thermosets has been demonstrated via associative exchange in the presence of a strong organic acid. Intending to introduce spatiotemporal control in the processability of polyurethane materials, in this work we investigate dynamic transcarbamoylation by the addition of a latent acidic catalyst into an aromatic and an aliphatic polyurethane network. We introduce three photoswitches based on merocyanine/spiropyran compounds which, upon exposure to visible light, undergo a reversible cyclization reaction, resulting in the formation of strong acids. The use of such photoswitches prevents the presence of a permanent acidic species in the network, which would eventually lead to its degradation over time. Importantly, due to the reversible nature of the cyclization reaction, the release of the acidic catalyst is not permanent, as evidenced by stress relaxation measurements. Overall, the evidence of the light-induced, acid-catalyzed transcarbamoylation is supported by stress relaxation, optical microscopy, and self-healing measurements.

Toward Circular Recycling of Polyurethanes: Depolymerization and Recovery of Isocyanates.

We report a depolymerization strategy to nearly quantitatively regenerate isocyanates from thermoplastic and thermoset polyurethanes (PUs) and then resynthesize PUs using the recovered isocyanates. To date, chemical/advanced recycling of PUs has focused primarily on the recovery of polyols and diamines under comparatively harsh conditions (e.g., high pressure and temperature), and the recovery of isocyanates has been difficult. Our approach leverages an organoboron Lewis acid to depolymerize PUs directly to isocyanates under mild conditions (e.g., ∼80 °C in toluene) without the need for phosgene or other harsh reagents, and we show that both laboratory-synthesized and commercially sourced PUs can be depolymerized. Furthermore, we demonstrate the utility of the recovered isocyanate in the production of second-generation PUs with thermal properties and molecular weights similar to those of the virgin PUs. Overall, this route uniquely provides an opportunity for circularity in PU materials and can add significant value to end-of-life PU products.

Upcycling of Carbon Fiber/Thermoset Composites into High‐Performance Elastomers and Repurposed Carbon Fibers

AbstractRecycling of carbon fiber‐reinforced polymer composites (CFRCs) based on thermosetting plastics is difficult. In the present study, high‐performance CFRCs are fabricated through complexation of aromatic pinacol‐cross‐linked polyurethane (PU−AP) thermosets with carbon fiber (CF) cloths. PU−AP thermosets exhibit a breaking strength of 95.5 MPa and toughness of 473.6 MJ m−3 and contain abundant hydrogen‐bonding groups, which can have strong adhesion with CFs. Because of the high interfacial adhesion between CF cloths and PU−AP thermosets and high toughness of PU−AP thermosets, CF/PU−AP composites possess a high tensile strength of >870 MPa. Upon heating in N,N‐dimethylacetamide (DMAc) at 100 °C, the aromatic pinacols in the CF/PU−AP composites can be cleaved, generating non‐destructive CF cloths and linear polymers that can be converted to high‐performance elastomers. The elastomers are mechanically robust, healable, reprocessable, and damage‐resistant with an extremely high tensile strength of 74.2 MPa and fracture energy of 149.6 kJ m−2. As a result, dissociation of CF/PU−AP composites enables the recovery of reusable CF cloths and high‐performance elastomers, thus realizing the upcycling of CF/PU−AP composites.

Upcycling of Carbon Fiber/Thermoset Composites into High-Performance Elastomers and Repurposed Carbon Fibers.

Recycling of carbon fiber-reinforced polymer composites (CFRCs) based on thermosetting plastics is difficult. In the present study, high-performance CFRCs are fabricated through complexation of aromatic pinacol-cross-linked polyurethane (PU-AP) thermosets with carbon fiber (CF) cloths. PU-AP thermosets exhibit a breaking strength of 95.5 MPa and toughness of 473.6 MJ m-3 and contain abundant hydrogen-bonding groups, which can have strong adhesion with CFs. Because of the high interfacial adhesion between CF cloths and PU-AP thermosets and high toughness of PU-AP thermosets, CF/PU-AP composites possess a high tensile strength of >870 MPa. Upon heating in N,N-dimethylacetamide (DMAc) at 100 °C, the aromatic pinacols in the CF/PU-AP composites can be cleaved, generating non-destructive CF cloths and linear polymers that can be converted to high-performance elastomers. The elastomers are mechanically robust, healable, reprocessable, and damage-resistant with an extremely high tensile strength of 74.2 MPa and fracture energy of 149.6 kJ m-2. As a result, dissociation of CF/PU-AP composites enables the recovery of reusable CF cloths and high-performance elastomers, thus realizing the upcycling of CF/PU-AP composites.

4D printed hydrogel scaffold with swelling-stiffening properties and programmable deformation for minimally invasive implantation.

The power of three-dimensional printing in designing personalized scaffolds with precise dimensions and properties is well-known. However, minimally invasive implantation of complex scaffolds is still challenging. Here, we develop amphiphilic dynamic thermoset polyurethanes catering for multi-material four-dimensional printing to fabricate supportive scaffolds with body temperature-triggered shape memory and water-triggered programmable deformation. Shape memory effect enables the two-dimensional printed pattern to be fixed into temporary one-dimensional shape, facilitating transcatheter delivery. Upon implantation, the body temperature triggers shape recovery of the one-dimensional shape to its original two-dimensional pattern. After swelling, the hydrated pattern undergoes programmable morphing into the desired three-dimensional structure because of swelling mismatch. The structure exhibits unusual soft-to-stiff transition due to the water-driven microphase separation formed between hydrophilic and hydrophobic chain segments. The integration of shape memory, programmable deformability, and swelling-stiffening properties makes the developed dynamic thermoset polyurethanes promising supportive void-filling scaffold materials for minimally invasive implantation.

Self‐healing and reprocessing of saturated hydroxyl‐terminated polybutadiene‐based networks enabled by dynamic covalent chemistry

AbstractHydroxyl‐terminated polybutadiene (HTPB) is found in many applications due to its ease of manufacturing, useful mechanical properties over a wide temperature range, and reactive hydroxyl chain ends. Typically, HTPB is crosslinked with isocyanates to form polyurethane thermosets. Limitations of this approach include the use of toxic isocyanates and the oxidative instability of backbone alkenes. In this work, saturated HTPB is used to form reprocessable covalent adaptable networks that are capable of stress relaxation and reprocessing, without relying on isocyanates or unstable alkenes. This approach introduces dynamic chemistry to the HTPB network via chain extension and subsequent crosslinking with 4‐methyl caprolactone (4mCL) and a novel bislactone crosslinker. Using benzenesulfonic acid (BSA) as a transesterification catalyst, stress relaxation times range from 150 to 8 min at temperatures of 70 to 100 °C. Despite crosslinking, these networks behave elastically, as evidenced by strain‐at‐break values of 93% for pristine samples, and dynamically, as shown by a strain‐at‐break of 72% after reprocessing the damaged samples. Shape reprogramming is also demonstrated by straining the crosslinked networks and heating to elevated temperatures where bond exchange occurs. These findings illustrate the advantageous properties that can be achieved by using cheap commodity building blocks to achieve dynamic properties. We anticipate that valorizing commodity polymers into reprocessable thermosets will be of utility in applications that lack other viable recycling pathways.

Experimental investigation on low-velocity impact behavior of glass, Kevlar, and hybrid composites with an elastomeric polyurethane matrix

Low-velocity impacts represent a critical dynamic condition for engineering structures. Combining two reinforcing fibers in a single matrix, i.e., hybridization, is considered a feasible way to improve composite performance. In this context, this paper presents an experimental work on composites with Kevlar and glass fabrics and a novel thermoset polyurethane matrix. The coupons are manufactured by vacuum infusion technique and low-velocity impact tests are carried out. First, the impact behavior of Kevlar and glass laminates of different thicknesses is assessed, and then impact tests are performed on different configurations of hybrid laminates, both symmetric and non-symmetric. For the non-symmetric specimens, impact tests were conducted on both sides of the stack. Load vs displacement curves are reported along with absorbed energy. To investigate the damage mechanism, the front, back, and cross-section views of the specimens are analyzed, and features related to the stacking sequences are discussed. Thermographic analyses are carried out on the impacted specimens to further analyze damage. The failure mechanisms are different from traditional epoxy composites and a hybridization effect is reported. The results evidence that the hybrid coupons are viable for structural applications, being capable of absorbing high-impact energies, in particular, non-symmetric hybrid laminates outperformed the Kevlar, glass, and symmetric ones, absorbing roughly 15% less energy for the highest energy impact.

Preparation and properties of block copolymerization thermoset polyurethane elastomers prepared in situ

Polyurethane elastomers are commonly employed as binder in propellant propellants and explosives due to their excellent mechanical properties. In-situ block copolymer synthesis of polyurethane for several advantages, including the elimination of the need for solvents in the reaction process, avoidance of excessive reaction steps, and minimizing the generation of byproducts. On the other hand, the in-situ method utilizes a low-cost prepolymer, making it highly suitable for large-scale production. Polyethylene glycol is currently one of the most widely used prepolymer in polymerization. PCL possesses favorable solubility and exhibits hydrophobicity with a low melting point, remarkably noteworthy are its thermoplastic properties, including a high decomposition temperature and a low glass transition temperature. This work employed an in-situ block copolymerization method to synthesize polyurethane elastomers and subsequently investigated their mechanical properties, surface structure, evolution over time, and thermal decomposition process. Polyurethane elastomers exhibited favorable mechanical properties, the elongation properties of these polyurethane systems herein are influenced by the proportion of hydrogen-bonded carbonyl groups within the urethane. The thermal decomposition of polyurethane elastomers can be divided into two distinct processes. Random scission of the polyurethane yielding PCL occurs under low-temperature conditions, while specific cleavage events take place at higher temperatures.

Synthesis and characterization of ammonium containing cyclocarbonates and polyurethanes there from

Using a three steps procedure a water soluble cationic bis-cyclocarbonate was prepared. Aminolysis was attempted in water but was not successful because of rapid hydrolysis of the starting cyclocarbonate in these basic conditions. Non-isocyanate polyurethanes (linear and crosslinked) were then prepared in acetonitrile with high yields. The resulting materials exhibited good thermal properties (Tdeg > 230 °C) and Tg > 60 °C. The presence of the cationic moiety led to hydrophilic materials with swelling rate of 45% and a plastification effect was observed with Tg decreasing down to −28 °C. Further, considering ammonium characteristics, antibacterial properties were assessed. Surface adhesion test showed a 99,9% inhibition towards gram-positive and gram-negative bacteria for the polyurethane thermoset. Acute toxicity was evaluated on the bis-cyclocarbonate oligomer confirming its innocuity in the studied concentration range.

Colorless, transparent, and shape memory polyurethane networks with high strength and recyclability

AbstractThe traditional thermoset polyurethanes (PUs) can be recycled through the dynamic carbamate bonds under specific conditions. However, there is limited research on the structural evolution and property changes of these PUs before and after recycling. Herein, we designed and fabricated a thermoset polyurethane network (PCU) using polyhexamethylene carbonate diol (PHMC‐diol) as soft segments, and isophorone diisocyanate (IPDI) and glycerol (GLY) as hard segments. Through careful formulation design, the PCU demonstrated colorless transparency (with up to 86% transmission), shape memory properties, and reprocessing capabilities, all achieved without introducing other dynamic covalent bonds. Furthermore, the PCU exhibited excellent mechanical properties, boasting a tensile strength of 59.2 ± 3.7 MPa and the elongation at break of 674.8 ± 37.3%. Of particular interest, the PCU samples displayed solid‐state plasticity and impressive shape memory performance, with shape fixity and recovery ratios exceeding 90%. Leveraging the combination of solid‐state plasticity and shape memory, the PCU samples demonstrated the ability to achieve arbitrary shape manipulations. Upon recycling, it was observed that the cross‐linking density of the PCU samples decreased, resulting in the formation of byproducts and subsequently leading to a reduction in tensile strength. Nonetheless, the PCU samples prepared in this study exhibit potential as thermadapt shape memory and recyclable materials.

Characterization of Turbid Biobased Polyurethane Thermosets by UV–Vis–NIR Spectroscopy Combined with Multivariate Data Analysis for In-Line Process Monitoring

Characterization of Turbid Biobased Polyurethane Thermosets by UV–Vis–NIR Spectroscopy Combined with Multivariate Data Analysis for In-Line Process Monitoring