materials Review Polyurethane Foams: Past, Present, and Future Nuno V. Gama 1 , Artur Ferreira 1,2 and Ana Barros-Timmons 1,* 1 CICECO—Aveiro Institute of Materials and Department of Chemistry, University of Aveiro–Campus Santiago, 3810-193 Aveiro, Portugal; [email protected] (N.V.G.); [email protected] (A.F.) 2 Escola Superior de Tecnologia e Gestão de Águeda-Rua Comandante Pinho e Freitas, No. 28, 3750-127 Águeda, Portugal * Correspondence: [email protected]; Tel.: +351-234370200; Fax: +351-234370985 Received: 12 August 2018; Accepted: 23 September 2018; Published: 27 September 2018 Abstract: Polymeric foams can be found virtually everywhere due to their advantageous properties compared with counterparts materials. Possibly the most important class of polymeric foams are polyurethane foams (PUFs), as their low density and thermal conductivity combined with their interesting mechanical properties make them excellent thermal and sound insulators, as well as structural and comfort materials. Despite the broad range of applications, the production of PUFs is still highly petroleum-dependent, so this industry must adapt to ever more strict regulations and rigorous consumers. In that sense, the well-established raw materials and process technologies can face a turning point in the near future, due to the need of using renewable raw materials and new process technologies, such as three-dimensional (3D) printing. In this work, the fundamental aspects of the production of PUFs are reviewed, the new challenges that the PUFs industry are expected to confront regarding process methodologies in the near future are outlined, and some alternatives are also presented. Then, the strategies for the improvement of PUFs sustainability, including recycling, and the enhancement of their properties are discussed. Keywords: polyurethane foams; sustainability; enhancement of properties; new processing methodologies 1. Polymeric Foams Materials such as plastic foams, foamed plastics, cellular plastics, or polymeric foams are materials that consist of a solid phase and a gas phase [1]. Polymer foams can be rigid, flexible, or elastomeric, and can be produced from a wide range of polymers, such as polyurethane (PU), polystyrene (PS), polyisocyanurate (PIR), polyethylene (PE), polypropylene (PP), poly(ethylene-vinyl acetate) (EVA), nitrile rubber (NBR), poly(vinyl chloride) (PVC), or other polyolefins, being the world foam production dominated by PU foams (PUFs), followed by PS and PVC foams [2,3]. The global market for polymeric foams was worth more than $100 billion in 2015, with sales of more than 22 million tons and a consumption of 25.3 million tons is expected by 2019 [4,5]. Being lightweight materials whose properties can be easily tuned, polymeric foams are the first choice for a wide range of applications such as: packaging, automotive, electronics, furnishing, footwear, aerospace, toys, food contact, or construction materials [3,6]. PUFs [7] are commonly used in comfort applications or as thermal and sound insulation materials, PS foams [8] are commonly used as food packaging, thermal, and sound insulation materials, and PVC foams [9] are commonly used as transport and construction materials. 2. Polyurethane Foams The first urethane was synthesized in 1849 by Wurtz [10]. Afterwards, in 1937, Otto Bayer synthesized PUs from the reaction between a polyester diol and a diisocyanate [10–13]. Indeed, this was a major breakthrough at the time, as it consisted of a new class of polymerization reaction called Materials 2018, 11, 1841; doi:10.3390/ma11101841 www.mdpi.com/journal/materials Materials 2018, 11, x FOR PEER REVIEW 2 of 34 Materials 2018, 11, x FOR PEER REVIEW 2 of 34 Materials 2018,, 11,, x 1841 FOR PEER REVIEW 22 of of 34 35 was a major breakthrough at the time, as it consisted of a new class of polymerization reaction called was a major breakthrough at the time, as it consisted of a new class of polymerization reaction called waspolyaddition, a major breakthrough which is also at known the time, as step as it polymerizationconsisted of a new [10,12]. class Nevertheless, of polymerization at first, reaction this polymer called polyaddition, which is also known as step polymerization [10,12]. Nevertheless, at first, this polymer polyaddition,was considered which useless is also [11]. known as step polymerization [10,12]. [10,12]. Nevertheless, Nevertheless, at first, first, this polymer polymer was considered useless [11]. was considered PUs are polymers useless that [[11].11]. are formed by the reaction between the OH (hydroxyl) groups of a polyol PUs are polymers that are formed by the reaction between the OH (hydroxyl) groups of a polyol withPUs the NCOare polymers (isocyanate that functional are formed group) by the groups reaction reaction of between an isocyanate, the OH and (hydroxy (hydroxyl) the namel) groups is associated of a polyol with with the NCO (isocyanate functional group) groups of an isocyanate, and the name is associated with withthe resulting the NCO urethane (isocyanate linkage functional [7,11,12,14]. group) Thisgroups reaction of an isocyanate, is exothermic, and and the nameleads isto associated associatedthe production with with the resulting urethane linkage [7,11,12,14]. This reaction is exothermic, and leads to the production thetheof urethane resulting groups urethaneurethane as linkagedescribedlinkage [[7,11,12,14].7 ,before11,12,14 and]. ThisThis illustrated reaction reaction in is isScheme exothermic, exothermic, 1 [7,10]. and and leads leads to to the the production production of of urethane groups as described before and illustrated in Scheme 1 [7,10]. ofurethane urethane groups groups as as described described before before and and illustrated illustrated in Schemein Scheme1[7 ,110 [7,10].]. Scheme 1. Reaction scheme of urethane production [10]. Scheme 1. Reaction scheme of urethane production [10]. SchemeScheme 1 1.. ReactionReaction scheme scheme of of urethane urethane production [10]. [10]. Where Riso is derived from the isocyanate monomer, while Rpolyol is derived from the polyol WhereWhere Riso isis derived derived from from the the isocyanate isocyanate monomer, monomer, while while RpolyolR is derivedis derived from the from polyol the component.Where Risoiso is derived from the isocyanate monomer, while Rpolyol ispolyol derived from the polyol component. component.polyolNowadays, component. PUs are used as everyday life products, being one of the most important class of Nowadays, PUs are used as everyday life products, being one of the most important class of polymersNowadays, that keep PUs PUs changing are are used the as qualityeveryday of thelife humanproduc products, ts,life being being[10]. Theone worldwideof the most consumption important class of PU of polymers that keep changing the quality of the human life [10]. The worldwide consumption of PU polymerswas estimated that keep at 60.5 changing billion USD the quality in 2017, of and the it humanwas predicted life [[10].10 ].to The be over worldwide 79 billion consumption USD by 2021 of [15]. PU was estimated at 60.5 billion USD in 2017, and it was predicted to be over 79 billion USD by 2021 [15]. wasIn 2016, estimated it represented at 60.5 billion nearly USD 9% inof 2017,the 2017, global and and it itconsumption wa wass predicted predicted of to toplastics be be over over [16]. 79 79 billion billionMoreover, USD USD asby by illustrated 2021 2021 [15]. [15]. In 2016, it represented nearly 9% of the global consumption of plastics [16]. Moreover, as illustrated InInin 2016,Figure it 1, represented the principal nearlynearly consumption 9%9% of of the the globalof global PUs consumption isconsumption in the form of ofof plastics foamsplastics [17]. [16 [16].]. Moreover, Moreover, as as illustrated illustrated in in Figure 1, the principal consumption of PUs is in the form of foams [17]. inFigure Figure1, the 1, the principal principal consumption consumption of PUsof PUs is in is thein the form form of foamsof foams [17 ].[17]. Elastomers; Adhesives and sealants; 6% Elastomers; Adhesives and sealants; 6% 6% Adhesives and sealants; 6% Elastomers;6% 6% Molded Coatings; foam;Molded 11% Coatings;3% foam;Molded 11% Coatings;3% foam; 11% 3% Others; Others;18% Flexible Others;18% foam;Flexible 31% 18% foam;Flexible 31% foam; 31% Rigid foam; Rigid25% foam; Rigid25% foam; 25% Figure 1. Global consumption of polyurethane (PU) in 2016 . FigureFigure 1.1. GlobalGlobal consumptionconsumption ofof polyurethanepolyurethane (PU)(PU) inin 2016.2016. Figure 1. Global consumption of polyurethane (PU) in 2016. Among PU consumption, PUFs correspond to 67% ofof globalglobal PUPU consumption.consumption. Furthermore, Among PU consumption, PUFs correspond to 67% of global PU consumption. Furthermore, sinceAmong the technology PU consumption, to produce PUFs is so well-established,correspond to 67% this of type global of foamsPU consumption. corresponds Furthermore,to half of the since the technology to produce is so well-established, this type of foams corresponds to half of the sincewhole the polymeric technology foam’s to produce market [[11].is11 so]. The well-established, main types of this PUFs type are of the foams flexible flexible corresponds foams and to rigid half foams;of the whole polymeric foam’s market [11]. The main types of PUFs are the flexible foams and rigid foams; wholenevertheless, polymeric other foam’s classificationsclassifications market [11]. cancan The be attributed main types to of PUFs, PUFs such are asthe flexible flexible flexible