materials Review Recent Advances in Fabrication of Non-Isocyanate Polyurethane-Based Composite Materials Piotr Stachak * , Izabela Łukaszewska , Edyta Hebda and Krzysztof Pielichowski Department of Chemistry and Technology of Polymers, Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Cracow, Poland; [email protected] (I.Ł.); [email protected] (E.H.); [email protected] (K.P.) * Correspondence: [email protected] Abstract: Polyurethanes (PUs) are a significant group of polymeric materials that, due to their outstanding mechanical, chemical, and physical properties, are used in a wide range of applications. Conventionally, PUs are obtained in polyaddition reactions between diisocyanates and polyols. Due to the toxicity of isocyanate raw materials and their synthesis method utilizing phosgene, new cleaner synthetic routes for polyurethanes without using isocyanates have attracted increasing attention in recent years. Among different attempts to replace the conventional process, polyaddition of cyclic carbonates (CCs) and polyfunctional amines seems to be the most promising way to obtain non-isocyanate polyurethanes (NIPUs) or, more precisely, polyhydroxyurethanes (PHUs), while primary and secondary –OH groups are being formed alongside urethane linkages. Such an approach eliminates hazardous chemical compounds from the synthesis and leads to the fabrication of polymeric materials with unique and tunable properties. The main advantages include better chemical, mechanical, and thermal resistance, and the process itself is invulnerable to moisture, which is an essential technological feature. NIPUs can be modified via copolymerization or used Citation: Stachak, P.; Łukaszewska, as matrices to fabricate polymer composites with different additives, similar to their conventional I.; Hebda, E.; Pielichowski, K. Recent counterparts. Hence, non-isocyanate polyurethanes are a new class of environmentally friendly Advances in Fabrication of polymeric materials. Many papers on the matter above have been published, including both original Non-Isocyanate Polyurethane-Based research and extensive reviews. However, they do not provide collected information on NIPU Composite Materials. Materials 2021, composites fabrication and processing. Hence, this review describes the latest progress in non- 14, 3497. https://doi.org/10.3390/ isocyanate polyurethane synthesis, modification, and finally processing. While focusing primarily ma14133497 on the carbonate/amine route, methods of obtaining NIPU are described, and their properties are Academic Editor: Janis¯ Andersons presented. Ways of incorporating various compounds into NIPU matrices are characterized by the role of PHU materials in copolymeric materials or as an additive. Finally, diverse processing methods Received: 17 February 2021 of non-isocyanate polyurethanes are presented, including electrospinning or 3D printing. Accepted: 19 June 2021 Published: 23 June 2021 Keywords: non-isocyanate polyurethanes; NIPU; copolymers; (nano)composites; hybrids; processing Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Polyurethanes (PUs) are an important class of polymers obtained through the addition polymerization reaction of diisocyanates and diols. Due to their extraordinary mechani- cal, chemical, and physical properties (elasticity, biocompatibility, resistance to abrasion, tensile strength), PUs are used in a wide range of applications. Polyurethanes are used Copyright: © 2021 by the authors. as rigid and flexible foams, namely, as coatings, glues, sealants, thermoplastic materials, Licensee MDPI, Basel, Switzerland. elastomers, or thermosetting resins [1,2]. The applications include, but are not limited to, This article is an open access article thermal insulation, textiles, bed mattresses and pillows, automotive parts, or scaffolds for distributed under the terms and tissue engineering. conditions of the Creative Commons According to the method developed by Otto Bayer in the 1950s [1,3], PUs are obtained Attribution (CC BY) license (https:// by a polyaddition reaction between aliphatic or aromatic isocyanate that contains at least creativecommons.org/licenses/by/ two reactive groups and oligomeric polyol, possessing two or more reactive hydrogen 4.0/). Materials 2021, 14, 3497. https://doi.org/10.3390/ma14133497 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 27 Materials 2021, 14, 3497 2 of 27 According to the method developed by Otto Bayer in the 1950s [1,3], PUs are ob- tained by a polyaddition reaction between aliphatic or aromatic isocyanate that contains at least two reactive groups and oligomeric polyol, possessing two or more reactive hy- atoms.drogen Lowatoms. molecular Low molecular diols or diols diamines or diamines are used are as used chain as extenders.chain extenders. Reactions Reactions are primarilyare primarily carried carried out in out the in presence the presence of the catalystof the catalyst and a suitable and a suitable solvent. solvent. Scheme 1Scheme shows 1 theshows conventional the conventional two-step two PU- synthesis—thestep PU synthesis first— stagethe first of the stage reaction of the consists reaction of consists forming of aforming prepolymer a prepolymer (oligomer (oligomer end-capped end with-capped reactive with moieties). reactive Amoieties). chain extender A chain is extender added to is theadded mixture to the in mixture the second in the step, second and finally, step, and polyurethane finally, polyurethane is formed [ 1is,2 formed]. [1,2]. SchemeScheme 1. 1.Synthesis Synthesis ofof conventionalconventional polyurethane through through a a two two-step-step method method by by reacting reacting diiso- diiso- cyanate,cyanate, polyol, polyol, and and low low molecular molecular weight weight chain chain extender extender [1 [1].]. However,However, side side reactions reactions accompany accompany the the main main process, process, especially especially when when moisture moisture is is presentpresent (Scheme (Scheme2). Simultaneously2). Simultaneously with with the formation the formation of urethane of urethane bonds, isocyanatebonds, isocyanate groups maygroups react may with react water, with which water, leads which to theleads formation to the formation of amine of and amine carbon and dioxide. carbon Thisdioxide. re- actionThis reaction is known is asknown a foaming as a foaming reaction, reaction, and while and it while is desired it is indesired the synthesis in the synthesis of foams, of itfoams, is unwanted it is unwanted in obtaining in obtaining polyurethane polyurethane elastomers elastomers [4]. Formed [4] amines. Formed further amines react further with isocyanate groups, and urea moieties are formed [5,6]. react with isocyanate groups, and urea moieties are formed [5,6]. The leading environmental issue of polyurethanes concerns the fact that isocyanate pre- The leading environmental issue of polyurethanes concerns the fact that isocyanate cursors show some toxicity [7]. The most frequently used isocyanates in the polyurethane precursors show some toxicity [7]. The most frequently used isocyanates in the polyure- industry are methylene diphenyl 4,40-diisocyanate (MDI) and toluene diisocyanate (TDI). thane industry are methylene diphenyl 4,4’-diisocyanate (MDI) and toluene diisocyanate Although the toluene diisocyanate or methyl diisocyanates utilized in PU manufactur- (TDI). Although the toluene diisocyanate or methyl diisocyanates utilized in PU manu- ing are considered less of an irritant than methyl isocyanate (used in the production of facturing are considered less of an irritant than methyl isocyanate (used in the produc- pesticides) due to their lower volatilities, all abovementioned compounds are labeled as tion of pesticides) due to their lower volatilities, all abovementioned compounds are la- CMR (carcinogen, mutagen, and reprotoxic), which is a serious concern, especially for beled as CMR (carcinogen, mutagen, and reprotoxic), which is a serious concern, espe- workers at polyurethane plants [8]. Lack of knowledge of potential health hazards con- cially for workers at polyurethane plants [8]. Lack of knowledge of potential health haz- cerning isocyanates may lead to more severe consequences. The main direct threats of humanards concerning exposure toisocyanates isocyanates may involve lead occupational to more severe asthma, consequences. respiratory The tract main disorders, direct sensitizationthreats of human of respiratory exposure tracts, to isocyanates or allergic involve reactions. occupational As potent asthma, sensitizers, respiratory isocyanates tract standdisorders, as leading sensitization causes of of occupational respiratory asthma tracts, aroundor allergic the world.reactions. Moreover, As potent it is sensitizers, likely that sensitizationisocyanates bystand isocyanates as leading through causes skin of exposureoccupational may asthma also be responsiblearound the forworld. isocyanate- Moreo- inducedver, it is asthma likely [that9,10 ].sensitization by isocyanates through skin exposure may also be re- sponsible for isocyanate-induced asthma [9,10]. Materials 2021, 14, 3497 3 of 27 Scheme 2. Urethane formation reaction and side reactions occurring during the synthesis of conven- tional polyurethanes [11]. Not only are isocyanates themselves hazardous but also the substrates used for their synthesis.