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...

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.

Non-isocyanate polyurethanes (NIPUs) based on fivemembered cyclic carbonates have emerged as the most promising alternative to replace more toxic, conventional polyurethanes. However, the low reactivity of five-membered cyclic carbonates has limited the preparation of one pot systems because of long curing times. This work focuses on the improvement and application of these materials as adhesives through the combination of NIPU chemistry and a sol−gel process that allows for curing by atmospheric humidity. The synthesis of NIPU prepolymers functionalized with (3-aminopropyl)triethoxysilane (APTES) is demonstrated, and their curing behavior and adhesion performance are investigated by means of rheological experiments and a lap-shear test, respectively. In spite of the ability of the alkoxysilane to cure under ambient conditions, our results show that the use of catalyst and elevated temperatures speeds up the curing process and leads to improved adhesion properties. Hence, it is demonstrated that the fastest curing and the best performance are achieved at 100 °C when acetic acid is employed as the catalyst. Finally, we demonstrate the importance of the soft (poly(propylene glycol)dicarbonate) to hard (resorcinol dicarbonate) ratio to achieve superior cohesion and adhesion properties in NIPU adhesives.

The preparation of non‐isocyanate polyurethanes (NIPUs) by polyaddition of (poly)cyclic carbonates to (poly)amines represents one of the most optimistic alternatives for replacing conventional polyurethanes prepared by the toxic isocyanate chemistry. However, the limited reactivity of conventional five membered cyclic carbonates even in the presence of catalysts restricts their industrial implementation. One way to mitigate this lack of reactivity is to combine with other chemistries to create hybrid‐NIPUs with superior performance. In this article the combination of the adhesive promoter, dopamine, and the fast‐curing promoter, an aminopropyl trimethoxysilane, is found to create a synergetic effect on the rheological and adhesive properties of NIPUs. After demonstrating the importance of adjusting soft/hard ratios to obtain lap‐shear strength adhesion values up to 21 MPa on stainless steel, these values are retained when adding dopamine and silane compounds. Importantly, the adhesive properties of NIPU are preserved at high temperature (T > 200 °C) for optimal compositions. Finally, adhesion tests on various substrates (polyamide, high density polyethylene, poly(methyl methacrylate), oak wood, and aluminum) show best performances on polar substrates confirming the strong interactions of hydroxyl groups of NIPU and dopamine.

Poly­(hydroxy urethane)­s (PHUs) based on 5-membered cyclic carbonates have emerged as sustainable alternatives to conventional isocyanate-based polyurethanes. However, while from the point of view of sustainability they represent an improvement, their properties are still not competitive with conventional polyurethanes. In this work, the potential of PHUs as reversible hot-melt adhesives is discussed. We found that with a judicious choice of reagents (i.e., the dicyclic carbonate and diamine), the detrimental hydrogen bonding between the soft segment of the chains and the pendant hydroxyl groups was partially avoided, thus imparting PHUs with hot-melt adhesion properties (i.e., adhesion at elevated temperatures and cohesiveness at a temperature lower than T g/T m). The importance of a balanced hard to soft segment ratio, along with the relevance of the chain extender in the final properties, is highlighted. Addition of aliphatic diamines (HMDA, 1,12-DAD) resulted in rubbery materials, while the employment of cycloaliphatic (CBMA) or aromatic ones (MXDA, PXDA) led to materials with hot-melt adhesive properties. The thermoreversibility of all compositions was assessed by rebonding specimens after lap-shear tests. Lap-shear strength values that were comparable to the virgin adhesives were observed. The breaking and reformation of hydrogen bonding interactions was demonstrated by FTIR measurements at different temperatures, as well as by rheological frequency sweep experiments. In order to mitigate the negative impact of the low molar mass PHUs and to enhance the service temperature of the adhesives, a hybrid PHU was prepared by adding a small amount of an epoxy resin, which acts as a cross-linker. These hybrid PHUs maintain the thermoreversibility displayed by thermoplastic PHUs while providing better adhesion at elevated temperatures. We believe that this work provides some important insights into the design of PHU-based hot-melt adhesives.