Toward Sustainable Polyurethane Alternatives: A Review of the Synthesis, Applications, and Lifecycle of Non-Isocyanate Polyurethanes (NIPUs)

In recent decades, scientific interest has increasingly focused on sustainable and green polymers. Within this context, considerable efforts have been devoted to the synthesis and exploration of eco-friendly non-isocyanate polyurethanes (NIPUs) as alternatives to conventional polyurethanes (PUs), solving the problem of isocyanate toxicity and other environmental problems that existed. This review article highlights the synthetic pathways of NIPUs and identifies the potential hazards associated with their production and end-of-life (EoL) stages. While in the literature there are several reviews regarding the synthesis of NIPUs, the current work distinguishes itself by providing a comprehensive summary of the latest research on NIPUs, with a particular focus on their lifecycle management, recyclability, and the challenges that hinder their scalability for industrial-level production. Advances in NIPU synthesis have made them strong candidates for a diverse range of applications. This review underscores the most notable examples of these advancements, emphasizing their potential to drive sustainable polymer development.

Modifying Polyvinylidene Chloride Resin with Fluorine Monomer and Cross-Linking Monomers

Modified polyvinylidene chloride (PVDC) resin was prepared using octafluoropentyl methacrylate and trimethylolpropane trimethacrylate as modifying monomers through seeded emulsion polymerization. The successful incorporation of octafluoropentyl methacrylate into the PVDC resin was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and XPS were utilized to investigate the element distribution in the modified monomer emulsion and the mechanism of monomer modification. The results demonstrated that the fluorine monomer was reacted in the resin, and mainly concentrated on the surface of the resin. The addition of octafluoropentyl methacrylate and trimethylolpropane trimethacrylate improved the water resistance of the resin. Compared to unmodified PVDC resin, the contact angle of the modified PVDC resin increased from 89.46° to 109.51°, and the water resistance at room temperature increased from 120 to 500 h. Furthermore, the modified resin exhibited excellent mechanical properties, thermal stability, and storage stability.

Innovations in applications and prospects of non-isocyanate polyurethane bioplastics

Currently, conventional plastics are necessary for a variety of aspects of modern daily life, including applications in the fields of healthcare, technology, and construction. However, they could also contain potentially hazardous compounds like isocyanates, whose degradation has a negative impact on both the environment and human health. Therefore, researchers are exploring alternatives to plastic which is sustainable and environmentally friendly without compromising its mechanical and physical features. This review study highlights the production of highly eco-friendly bioplastic as an efficient alternative to non-biodegradable conventional plastic. Bioplastics are produced from various renewable biomass sources such as plant debris, fatty acids, and oils. Poly-addition of di-isocyanates and polyols is a technique employed over decades to produce polyurethanes (PUs) bioplastics from renewable biomass feedstock. The toxicity of isocyanates is a major concern with the above-mentioned approach. Novel green synthetic approaches for polyurethanes without using isocyanates have been attracting greater interest in recent years to overcome the toxicity of isocyanate-containing raw materials. The polyaddition of cyclic carbonates (CCs) and polyfunctional amines appears to be the most promising method to obtain non-isocyanate polyurethanes (NIPUs). This method results in the creation of polymeric materials with distinctive and adaptable features with the elimination of harmful compounds. Consequently, non-isocyanate polyurethanes represent a new class of green polymeric materials. In this review study, we have discussed the possibility of creating novel NIPUs from renewable feedstocks in the context of the growing demand for efficient and ecologically friendly plastic products.

Development of BioPolyurethane Coatings from Biomass-Derived Alkylphenol Polyols-A Green Alternative

Bio-based polyols were obtained from the thermochemical liquefaction of two biomass feedstocks, pinewood and Stipa tenacissima, with conversion rates varying between 71.9 and 79.3 wt.%, and comprehensively characterized. They exhibit phenolic and aliphatic moieties displaying hydroxyl (OH) functional groups, as confirmed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and nuclear magnetic resonance spectroscopy (NMR) analysis. The biopolyols obtained were successfully employed as a green raw material to produce bio-based polyurethane (BioPU) coatings on carbon steel substrates, using, as an isocyanate source, a commercial bio-based polyisocyanate-Desmodur^® Eco N7300. The BioPU coatings were analyzed in terms of chemical structure, the extent of the reaction of the isocyanate species, thermal stability, hydrophobicity, and adhesion strength. They show moderate thermal stability at temperatures up to 100 °C, and a mild hydrophobicity, displaying contact angles between 68° and 86°. The adhesion tests reveal similar pull-off strength values (ca. 2.2 MPa) for the BioPU either prepared with pinewood and Stipa-derived biopolyols (BPUI and BPUII). Electrochemical impedance spectroscopy (EIS) measurements were carried out on the coated substrates for 60 days in 0.05 M NaCl solution. Good corrosion protection properties were achieved for the coatings, with particular emphasis on the coating prepared with the pinewood-derived polyol, which exhibited a low-frequency impedance modulus normalized for the coating thickness of 6.1 × 10¹⁰ Ω cm at the end of the 60 days test, three times higher than for coatings prepared with Stipa-derived biopolyols. The produced BioPU formulations show great potential for application as coatings, and for further modification with bio-based fillers and corrosion inhibitors.

Ultrasonic nondestructive evaluation of the bonding strength of polyurethane coatings based on feedforward comb filtering effect

Because of the ability to protect and decorate materials, polyurethane coatings (PU) are being applied more and more extensively in the transportation, furniture, automotive, and electronic industries. However, it is difficult to effectively evaluate the bonding quality of coatings by conventional destructive methods for high-volume products in industrial applications. In this work, an ultrasonic nondestructive method based on the feedforward comb filtering effect was proposed to assess the bonding strength of the PU coatings. The coating and substrate interface can be modeled as a distributed spring system. The phase of the reflected ultrasonic wave is highly related to the interfacial stiffness. The superimposed signals can be well analyzed based on the comb filtering effect, and an index called the ratio of notch frequencies to interval frequencies (NIR) was constructed to represent the phase difference. Finite element simulations were conducted to show the index's construction method and variation with different interfacial stiffnesses. Immersed ultrasonic testing experiments with a transducer of 39.43 MHz were then conducted on PU coatings on two kinds of substrates made of aluminum and magnesium alloys. Each specimen was scanned as a grid to mimic the procedure of the well-known cross-cut test. The notch frequencies of the scanned received signals were applied to obtain the NIR index. To avoid the interference of the thickness of coatings, the decimal part of NIR was further proposed to link with the bonding strength obtained by the cross-cut test. This index changes accordingly with aluminum- and magnesium-based coatings' bonding strength. The results prove that the proposed ultrasonic method based on the feedforward comb filter effect is high potential to assess the bonding strength of coatings nondestructively.

Sustainable cycloaliphatic polyurethanes: from synthesis to applications

Polyurethanes (PUs) are a versatile and major polymer family, mainly produced via polyaddition between polyols and polyisocyanates. A large variety of fossil-based building blocks is commonly used to develop a wide range of macromolecular architectures with specific properties. Due to environmental concerns, legislation, rarefaction of some petrol fractions and price fluctuation, sustainable feedstocks are attracting significant attention, e.g., plastic waste and biobased resources from biomass. Consequently, various sustainable building blocks are available to develop new renewable macromolecular architectures such as aromatics, linear aliphatics and cycloaliphatics. Meanwhile, the relationship between the chemical structures of these building blocks and properties of the final PUs can be determined. For instance, aromatic building blocks are remarkable to endow materials with rigidity, hydrophobicity, fire resistance, chemical and thermal stability, whereas acyclic aliphatics endow them with oxidation and UV light resistance, flexibility and transparency. Cycloaliphatics are very interesting as they combine most of the advantages of linear aliphatic and aromatic compounds. This original and unique review presents a comprehensive overview of the synthesis of sustainable cycloaliphatic PUs using various renewable products such as biobased terpenes, carbohydrates, fatty acids and cholesterol and/or plastic waste. Herein, we summarize the chemical modification of the main sustainable cycloaliphatic feedstocks, synthesis of PUs using these building blocks and their corresponding properties and subsequently present their major applications in hot-topic fields, including building, transportation, packaging and biomedicine.

Cellulosic fabrics modified with polyphosphonium chitosan hydrazone-TiO2-Ag nanobiocomposites for multifunctional applications

Bionanocomposites (BNC1,2) of binary (PPCH-Ag) and ternary (PPCH-TiO₂-Ag) (PPCH = polyphosphonium chitosan-hydrazone) have been synthesized and immobilized on cellulosic fabrics (CFs) using an environmentally friendly single-step in situ methodology. The results of FTIR, TGA, EDX, SEM, and TEM investigations showed that PPCH and its BNCs were successfully formed on the surface layer of fabrics. Moreover, the BNC2-coated cloth exhibited a superhydrophobic behavior as revealed from the values of water contact angle (WCA) 152.1° and slide angle (SA) 8.7°. The cytotoxicity experiments on epithelial cells confirmed the safety of treated fabrics for human cells. The antimicrobial capabilities of the BNCs-treated textiles were greatly enhanced, with a small preference for BNC1-coated fabric, as compared to the native or other treated fabrics. In contrast, the BNC2-coated fabric demonstrated the highest anti-UV protection capabilities as indicated by its great capacity to reduce the UV transmission (UV-A, 2.1 %; UV-B, 1.8 %) as well as its UPF value (49.2). The durability tests revealed the high resistance of BNC2-CF against harsh washing conditions and their acquired functions sustainability up to 20 washing cycles.

Synthesis of Nonisocyanate Poly(hydroxy)urethanes from Bis(cyclic carbonates) and Polyamines

Nonisocyanate polyurethane materials with pending alcohol groups in the polymeric chain were synthesized by polyaddition reaction of bis(cyclic carbonates) onto diamines. For the platform molecule, 1,4-butanediol bis(glycidyl ether carbonate) (BGBC, 1) was used. The polyaddition reaction of 1 onto a wide range of diamines with different electronic and physical properties was explored. All PHUs were obtained quantitatively after 16 h at 80 °C temperature in MeCN as solvent. The low nucleophilicity of L-lysine has proven unable to ring-open the cyclic carbonate and, thus, no reaction occurred. The addition of DBU or TBD as the catalyst was tested and allows the obtention of the desired PHU. However, the presence of strong bases also led to the formation of polyurea fragments in the new PHU. The different poly(hydroxyurethane) materials were characterized using a wide range of spectroscopic techniques such as NMR, IR, MALDI-ToF, and using GPC studies. The thermal properties of the NIPUs were investigated by DSC and TGA analyses. Moreover, reactions employing different monomer ratios were performed, obtaining novel hydroxycarbamate compounds. Finally, sequential and one-pot experiments were also carried out to synthesize the PHUs polymers in one-step reaction.

E-waste derived silica-alumina for eco-friendly and inexpensive Mg-Al-Ti photocatalyst towards glycerol carbonate (electrolyte) synthesis: Process optimization and LCA

Valorization ofe-waste, i.e. waste printed circuit board (WPCB) through mechano-chemical activation to obtain silica as the catalyst support and alumina as the catalyst precursor for eco-friendly synthesis of inexpensive highly proficient photocatalyst has been explored. The WPCB derived silica-supported layered double oxide photocatalyst (MATLS_W) and its counterpart (MATLS_C) involving commercial silica and alumina precursors were synthesized through the wet-impregnation method under energy-efficient solar simulated quartz halogen lamp (SSQHL) irradiations to improve its photocatalytic properties compared to conventional methods. The prepared MATLS_Wpossessed a significantly low band-gap-energy (1.58 eV) that rendered efficient photocatalysis in the green-synthesis of glycerol carbonate (GC) (an effective electrolyte). The catalytic performance of the optimal MATLS_Wresulted in a superior yield of GC (98.68%) compared to that rendered by MATLS_Ccatalyst (GC yield: 96.56%) at optimal process conditions. Detailed life cycle assessment (LCA) of the entire process (deploying Ecoinvent 3.5 database) dictated conducive environmental impacts concerning 1 kg GC synthesis alongside a scale-up study for 1 MT GC synthesis encompassing silica-alumina extraction from WPCB, MATLS_W preparation, and employment of SSQHL-radiated batch reactor (SSQHLBR) (56.64% less energy consumption than conventional). The overall process deploying the novel MATLS_Win conjunction with the effectual reactor demonstrated superiority over the conventional GC synthesis process through appreciable reductions of environmental impact parameters, namely GWP, FDP, and HTP by 5.78%, 3.60%, and 5.72% respectively. The developed green process for e-waste utilization can procreate an effective waste management protocol towards a cleaner world.

Trends in non-isocyanate polyurethane (NIPU) development

The transition towards safer and more sustainable production of polymers has led to a growing body of academic research into non-isocyanate polyurethanes (NIPUs) as potential replacements for conventional, isocyanate-based polyurethane materials. This perspective article focuses on the opportunities and current limitations of NIPUs produced by the reaction between biobased cyclic carbonates with amines, which offers an interesting pathway to renewable NIPUs. While it was initially thought that due to the similarities in the chemical structure, NIPUs could be used to directly replace conventional polyurethanes (PU), this has proven to be more challenging to achieve in practice. As a result, and in spite of the vast amount of academic research into this topic, the market size of NIPUs remains negligible. In this perspective, we will emphasize the main limitations of NIPUs in comparison to conventional PUs and the most significant advances made by others and us to overcome these limitations. Finally, we provide our personal view of where research should be directed to promote the transition from the academic to the industrial sector.

Ag immobilized lignin-based PU coating: A promising candidate to promote the mechanical properties, thermal stability, and antibacterial property of paper packaging

A lignin-based PU coating was prepared for paper-based green packaging. Two representative diisocyanate were used to prepare the coatings. Due to the rigid aromatic, the physical properties of the TDI system reached the maximum below the lignin content of 40%. The HDI that contains flexible aliphatic chains alleviated the brittleness of coating, and it showed physical advantages when the lignin content was more than 50%. Owing to the high lignin content, the coating presented enhanced thermal stability. After coated with the lignin-based PU coatings, the dry tensile strength of coated paper was improved by 126%. Amazingly, the wet strength was increased from 0.31 to 12.6 MPa with an improvement nearly 40 times. Based on the coordination of lignin, Ag⁺ was introduced into the PU matrix, which imparted the coating with excellent antibacterial ability. The colony forming units of E. coli and S. aureus were both less than 1. However, no inhibition halo was observed, which indicated that the Ag was firmly anchored on the coating and the antibacterial ability is only available when the bacterial contact the coating surface. The lignin-based PU coating with favorable sustainability and properties shows great potential in paper-based green packaging fields.

A Brief Introduction to the Polyurethanes According to the Principles of Green Chemistry

Polyurethanes are most often called “green” when they contain natural, renewable additives in their network or chemical structure, such as mono- and polysaccharides, oils (mainly vegetable oils), polyphenols (e.g., lignins, tannins), or various compounds derived from agro-waste white biotechnology (Principle 7). This usually results in these polyurethanes obtained from less hazardous substrates (Principle 4). Appropriate modification of polyurethanes makes them susceptible to degradation, and the use of appropriate processes allows for their recycling (Principle 10). However, this fulfilment of other principles also predisposes them to be green. As in the production of other polymer materials, the synthesis of polyurethanes is carried out with the use of catalysts (such as biocatalysts) (Principle 9) with full control of the course of the reaction (Principle 11), which allows maximization of the atomic economy (Principle 2) and an increase in energy efficiency (Principle 6) while minimizing the risk of production waste (Principle 1). Moreover, traditional substrates in the synthesis of polyurethanes can be replaced with less toxic ones (e.g., in non-isocyanate polyurethanes), which, at the same time, leads to a non-toxic product (Principle 3, Principle 5). In general, there is no need for blocking compounds to provide intermediates in the synthesis of polyurethanes (Principle 8). Reasonable storage of substrates, their transport, and the synthesis of polyurethanes guarantee the safety and the prevention of uncontrolled reactions (Principle 12). This publication is a summary of the achievements of scientists and technologists who are constantly working to create ideal polyurethanes that do not pollute the environment, and their synthesis and use are consistent with the principles of sustainable economy.

Fatty Acids and their Derivatives as Renewable Platform Molecules for the Chemical Industry

Oils and fats of vegetable and animal origin remain an important renewable feedstock for the chemical industry. Their industrial use has increased during the last 10 years from 31 to 51 million tonnes annually. Remarkable achievements made in the field of oleochemistry in this timeframe are summarized herein, including the reduction of fatty esters to ethers, the selective oxidation and oxidative cleavage of C-C double bonds, the synthesis of alkyl-branched fatty compounds, the isomerizing hydroformylation and alkoxycarbonylation, and olefin metathesis. The use of oleochemicals for the synthesis of a great variety of polymeric materials has increased tremendously, too. In addition to lipases and phospholipases, other enzymes have found their way into biocatalytic oleochemistry. Important achievements have also generated new oil qualities in existing crop plants or by using microorganisms optimized by metabolic engineering.

Starch-based isocyanate- and non-isocyanate polyurethane hybrids: A review on synthesis, performance and biodegradation

The challenges related to the persistence of plastics in natural ecosystems fostered strong interest in developing biodegradable bioplastics. Among natural biopolymers, starch gained both academic and industrial interest owing to its impressive physicochemical properties. The use of starch in production of polyurethane (PU) composites not only yields PUs with outstanding mechanical properties but also makes the final PU products biodegradable. The hydrophilic nature of starch limits its dispersion in hydrophobic PU polymers, although it is a significant benefit in creating starch-embedded non-isocyanate polyurethane (NIPU) composites. We present a comprehensive overview to highlight important strategies that are used to improve the compatibility of starch with various PU matrices. This review also gives an overview of the recent advances in the synthesis of starch-NIPU hybrids. Moreover, we aim to deliver critical insight into strategies that boost the biodegradation characteristics of PUs along with a discussion on various methods to assess their biodegradation.

Poly(hydroxyurethane) Adhesives and Coatings: State-of-the-Art and Future Directions

Polyurethane (PU) adhesives and coatings are widely used to fabricate high-quality materials due to their excellent properties and their versatile nature, which stems from the wide range of commercially available polyisocyanate and polyol precursors. This polymer family has traditionally been used in a wide range of adhesive applications including the bonding of footwear soles, bonding of wood (flooring) to concrete (subflooring), in the automotive industry for adhering different car parts, and in rotor blades, in which large surfaces are required to be adhered. Moreover, PUs are also frequently applied as coatings/paints for automotive finishes and can be applied over a wide range of substrates such as wood, metal, plastic, and textiles. One of the major drawbacks of this polymer family lies in the use of toxic isocyanate-based starting materials. In the context of the REACH regulation, which places restrictions on the use of substances containing free isocyanates, it is now urgent to find greener routes to PUs. While non-isocyanate polyurethanes (NIPUs) based on the polyaddition of poly(cyclic carbonate)s to polyamines have emerged in the past decade as greener alternatives to conventional PUs, their industrial implementation is at an early stage of development. In this review article, recent advances in the application of NIPUs in the field of adhesives and coatings are summarized. The article also draws attention to the opportunities and challenges of implementing NIPUs at the industrial scale.

Superhydrophobic cotton fabrics coated by chitosan and titanium dioxide nanoparticles with enhanced antibacterial and UV-protecting properties

Superhydrophobic cotton fabrics were fabricated using chitosan/titanium dioxide (TiO₂) nanocomposites. Morphology results revealed that the fabric's surface was utterly coated by the nanoparticles leading to the formation of a highly packed nano-scale structure in the case of superhydrophobic coating. X-ray photoelectron spectroscopy results also proved that TiO₂ nanoparticles were highly adsorbed onto the fabric's top layer. Durability of the superhydrophobic coating was investigated by immersing the fabric into harsh solutions and also by subjecting the fabric to sonication. The results showed the high resistance of the superhydrophobic fabric against harsh conditions. The nanocomposite-coated fabrics were found to exhibit promising UV-protecting properties especially for the superhydrophobic fabric which showed around 80% enhancement in the UV protecting properties as compared with the uncoated fabric. The bacterial adhesion results revealed that the combination of chitosan and TiO₂ results in high antibacterial properties against E. coli and S. aureus bacteria. The bacterial reduction percentages were further increased to 99.8 and 97.3% against E. coli and S. aureus, respectively, once the superhydrophobic character was also induced to the fabrics. The developed nanocomposite coated fabrics exhibited promising potential to be used as antibacterial and self-cleaning garments in hospital-related applications.

A novel catalytic two-step process for the preparation of rigid polyurethane foams: synthesis, mechanism and computational studies

Synthesis of polyurethane foams catalyzed by sodium chloride both in prepolymer preparation and elongation step. The reaction mechanism involves the role of a proton shuttle between two terminal OH groups of PEG 400.

Fully Renewable Non-Isocyanate Polyurethanes via the Lossen Rearrangement

In this work, a straightforward and efficient synthesis approach to renewable non-isocyanate polyurethanes (NIPUs) is described. For this purpose, suitable and renewable carbamate monomers, possessing two double bonds, are synthesized from hydroxamic fatty acid derivatives via the Lossen rearrangement in a one-step synthesis, and sustainable dithiols are synthesized from dialkenes derived from renewable feedstock (i.e., limonene and 1,4-cyclohexadiene). Subsequently, the comonomers are polymerized with the highly efficient thiol-ene reaction to produce NIPUs with M_n values up to 26 kg mol^-1 bearing thioether linkages. The main side product of the Lossen rearrangement, a symmetric urea, can also be polymerized in the same fashion. Important in the view of sustainability, the monomer mixture can also be used directly, without separation. The obtained polymers are characterized by NMR, attenuated total reflection-infrared spectroscopy, differential scanning calorimetry, and size exclusion chromatography.

Water-based non-isocyanate polyurethane-ureas (NIPUUs)

This review aims at discussing the achievements and the remaining challenges in the development of water-soluble NIPUUs, NIPUUs-based hydrogels and water-borne NIPUU dispersions.

Advances in the use of CO2 as a renewable feedstock for the synthesis of polymers

Carbon dioxide offers an accessible, cheap and renewable carbon feedstock for synthesis. Current interest in the area of carbon dioxide valorisation aims at new, emerging technologies that are able to provide new opportunities to turn a waste into value. Polymers are among the most widely produced chemicals in the world greatly affecting the quality of life. However, there are growing concerns about the lack of reuse of the majority of the consumer plastics and their after-life disposal resulting in an increasing demand for sustainable alternatives. New monomers and polymers that can address these issues are therefore warranted, and merging polymer synthesis with the recycling of carbon dioxide offers a tangible route to transition towards a circular economy. Here, an overview of the most relevant and recent approaches to CO2-based monomers and polymers are highlighted with particular emphasis on the transformation routes used and their involved manifolds.