A Review of the Sustainable Approaches in the Production of Bio-based Polyurethanes and Their Applications in the Adhesive Field

Abstract

On account of the irreversible environmental damage caused by the utilization of non-renewable raw materials in industrial production, since the end of the twentieth century, the interest in replacing the traditionally applied petroleum-based starting compounds in the polyurethane production by more sustainable feedstocks has grown enormously. Such pursuit of Green Chemistry has been fostered by the implementation of a set of national and international initiatives and stricter regulations, especially in the field of adhesives. In this respect, the latest advances in the production of bio-based polyurethanes are collected in this review. Thus, after a brief introduction to this subject and main tendencies towards the production of more sustainable polyurethanes, the first section reviews the feasibility of manufacturing polyurethanes from a range of natural platforms, including lignocellulosic biomass and vegetable oils, whether modified or in their original form, along with some industrial wastes. Afterwards, the hitherto considered synthetic routes for the preparation of greener polyurethanes are assessed, encompassing waterborne, radiation-curable and non-isocyanate polyurethane techniques. Finally, the last section focuses on the research advancement on the synthesis, properties and different uses of bio-based polyurethanes specifically implemented in the field of adhesives.

Towards the Green Chemistry in Polyurethane Production

Under the stimulus of high molecular weight polyamides, auspiciously synthesized by E.I. DuPont, Otto Bayer and collaborators, through the genuine discovery of the polycondensation reaction between alcohols and isocyanate by Wûrtz, succeeded in developing polyurethane fibers in 1937 [1], aimed to compete with commercialized nylon-based counterparts. However, they were unaware of the rapid expansion such materials would experience, representing a milestone in the polymer chemistry. Nonetheless, it was not until the recognition of their exceptional elasticity by DuPont and ICI in 1940 when their industrial manufacturing commenced, which would expand towards unimaginable limits [2]. Indeed, the worldwide average annual polyurethane consumption has risen from 12.9 up to 16.1 Mt from 2011 to 2016, and according to the worldwide statistical study conducted by The Statistics Portal [3], a growth of 20% is envisaged until 2021.

The conventional industrial production of urethane polymers deeply relies on petroleum stocks. However, as a consequence of the exhaustion of crude reserves, its subsequent floating price, along with the existing environmental awareness, the present paradigm in the polyurethane research has shifted towards the exploration of new synthetic routes leading to renewable bio-based products [4].

In general, the term “bio-based products” comprises any material derived from natural and renewable resources, but it entails neither biodegradability nor biocompatibility. Indeed, biodegradable polyurethanes, typically intended to temporary biomedical applications, are required to contain linkages ready to degrade under biological circumstances, also showing biocompatible properties, i.e. easy metabolization and lacking toxicity, capable of playing their role without causing adverse effects in the living organism [5].

In this sense, considering polyol building blocks, enzymatic hydrolysis of the ester bond in polyester-polyols makes them more appealing for the production of biodegradable polyurethanes [6]. Thus, polyester polyols like polycaprolactone [7], polylactic acid [8], and polyacrylic or polyglycolic acids [9, 10] have demonstrated not to produce toxic decomposition products. Additionally, polycarbonate polyols can also be proposed as biocompatible polyurethane precursors, as it has been shown by Zhu et al. [11] when assessing the biostability of poly(1,6-hexanediol) carbonate diol-based polyurethanes. Furthermore, as shown in the investigations of Calvo-Correas et al. [12, 13], Li. et al. [14] and Basterretxea et al. [15], l-lysine and dimeryl diisocyanates exhibited suitable biocompatible and biodegradable behaviour.

Moreover, the renewability and biocompatibility in the polyurethane network can be raised by means of chain extenders, as can be found in the investigation of Abdollahi et al. [6], who conducted the prepolymer chain extension with different combinations of 1,4-butanediol and curcumin aiming to preserve the polyurethane biocompatibility, while Kavanaugh et al. [16] combined the use of biodegradable polyols (poly-ε-caprolactone) with a series of osteogenically active molecules in pursuit of medical regeneration, evincing the ability of β-glycerol phosphate to act as chain extender in the production of cytocompatible segmented polyurethanes. Additionally, Opera et al. [17] succeeded in improving the biodegradation of polyurethane composites owing to the fungal susceptibility of ester group located in the vegetable oil (viz. castor oil) chain extender and the cellulose filler, strongly bonded to the urethane network through covalent bonds.

To summarize, the main focus of the current work is to review the recent advances in the partial or complete replacement of non-renewable raw materials by new eco-friendly alternatives, according to the concept of “sustainability”, which in agreement with Dubé and Salehpour [18] means “the ability to meet the current needs without jeopardizing the ability of future generations to meet their own needs”. Nevertheless, due to the greater difficulties in replacing non-sustainable isocyanates, most of the researchers have devoted their investigations to the development of polyurethanes with improved renewable content considering the use of lignocellulosic biomass, polysaccharides and natural oils as polyol sources. In the following sections the production of bio-sourced polyurethanes, not only from more renewable platforms but also by dint of the implementation of greener synthetic routes, are reported.

New Alternative Feedstocks: Polyols and Polyisocyanates

Lignocellulosic Materials and Derived Compounds as Polyol Precursors

Lignocellulose represents the most copious natural occurring biomaterial, which, together with its non-edible nature and significant hydroxyl functionality, confers its particular appeal as a renewable platform for the polyurethane production [19]. Lignocellulosic materials comprehend cellulose, hemicellulose and lignin, as the main compounds among other minor constituents (tannins, proteins, ash, etc.), whose composition is not evenly distributed, but strongly depends on the biomass’ nature [20].

Due to the fact that lignin is primarily obtained as a byproduct in the paper industry, the still reduced market niche and its reactive and hydroxyl-groups bearing character, lignin appears as a potential precursor for the production of polyurethanes [21]. As a consequence of the steric hindrance, lignocellulosic biomass may be submitted to different modifications to improve the accessibility of the hydroxyl groups to further react with isocyanate moieties [22]. The predominant chemical modifications to do that can be classified into liquefaction and oxypropylation, which will be described in the next sections. Nevertheless, aimed to avoid the adverse impact on the competitiveness of the bio-based polyols emerging from the energy loss and the inherent economic cost associated with the implementation of such additional steps [22], some researchers have addressed the development of polyurethanes from unmodified polysaccharides. With this aim, Araujo and Pasa [23] dedicated their study to the direct utilization of residual guaiacyl-syringyl-based lignin-containing residual polysaccharide resulting from the Eucalyptus tar distillation, so-called biopitch, in combination with different proportions of hydroxyl-terminated polybutadiene for the preparation of elastomeric polyurethanes. Additionally, Griffini et al. [22] demonstrated the effectiveness of polyurethane film preparation from fractionated softwood Kraft lignin, revealing the improved degree of sustainability of such coatings through a simplified synthetic route. Furthermore, Gallego et al. [24, 25] extended the application of lignocellulosic biomass in their natural form to the achievement of bio-inspired lubricating greases, preparing NCO-terminated thickening agents through a controlled condensation reaction of HDI not only with Kraft cellulose pulp but also with some derivatives, such as methylcellulose [26, 27] or (methyl) 2-hydroxyethyl cellulose [28]. More recently, Borrero-López et al. [29] first functionalized alkali Kraft lignin with a wide selection of aliphatic and aromatic isocyanates to evaluate the curing kinetics while analyzing the evolution of the viscoelastic response of the ensuing castor oil-based polyurethanes. Consequently, the most suitable modifier from that research in terms of the ultimate viscoelastic properties, the aliphatic 1,6-hexamethylene diisocyanate, was applied in further investigations [30, 31] as the crosslinking agent in oily medium in order to develop solid-state fermented wheat straw soda lignin for lubricating purposes.

Liquefaction

This treatment is based on the solvolysis of lignocellulosic biomass at temperatures within the range of 150–250 °C, leading to smaller hydroxyl-bearing molecules by dint of the utilization of polyhydric alcohols (ethylene or polyethylene glycol) or cyclic carbonates (ethylene carbonate) as the liquefaction solvents, and usually an acid catalyst [32]. Besides that, in order to prepare polyurethanes from long-chain polyfunctional alcohols, the resulting liquified units are typically submitted to a further transesterification producing natural polyester polyols, as reported by Sinha and coworkers [33, 34] and Patel et al. [35], in which the glycosides resulting from an acid liquefaction (α-D-glycoside and β-D-glycoside) with sulfuric and/or p-toluene sulfonic acids were esterified with oils or fatty acids at low pressure and high temperatures (220–250 °C), using lithium hydroxide as catalyst (Fig. 1). Similarly Zhou et al. [36] reported such liquefaction-transesterification procedure when partially substituting PEG400 with liquified banana pseudo-stem to prepare polyurethane for adhesion purposes.

Fig. 1

Liquefaction of cellulose with a subsequent transesterification

This solvolytic reaction is an effective process to increase both the functionality and sustainable nature of the starting polyols, notwithstanding the required optimization of the reaction conditions (temperature, time, catalyst and biomass to solvent weight ratio) aimed to mitigate the occurrence of secondary reactions and to reduce to the minimum possible the remaining solid content. Thus, some studies [37] intended to optimize, in terms of biomass conversion, the experimental conditions (temperature, time and catalyst concentration) to obtain liquified polyols from coffee grounds, accomplishing a 70% of yield at 160 °C, 80 min and sulfuric acid at 4% (w/w).

Despite the liquefaction modification can be conducted through the use of a series of solvents, as Liang et al. [38] showcased when evaluated the kinetics of this procedure on different feedstocks, the most challenging issue is to decrease or even eliminate its dependency on petroleum derivatives. On the other hand, as a consequence of the rapid expansion of the biodiesel industry, in which crude glycerol is produced as a byproduct, several studies suggest the incorporation of crude glycerol as solvent for the liquefaction of, inter alia, the solid residues of the saccharification of sunflower stalk [39] and empty fruit bunches [40], achieving a yield of 59% and 47.9%, respectively, in the production of bio-based polyols.

Oxypropylation

Unlike liquefaction, when optimized, oxypropylation is a chemical reaction capable of incorporating more readily available hydroxyl groups, avoiding the utilization of any solvent while simultaneously maintaining the biomass functionality [41]. This modification consists of a base-catalyzed grafting of propylene oxide onto lignocellulosic materials under high pressure and temperature (650–1820 kPa and 100–200 °C) (Fig. 2). As a result of the incorporation of short propylene oxide chains to lignocellulosic biomass, the starting solid material can be converted into a viscous polyol [42].

Fig. 2

Oxypropylation of lignocellulosic biomass

As related to any chemical transformation, some side reactions can be found yielding not only to partially oxypropylated biomass but also polypropylene oxide homopolymers, which, however, may take part in the chain extension of the polyurethane prepolymer [42]. Bearing this fact in mind, and due to the high-pressure requirements, the application of oxypropylation to produce bio-sourced polyurethane may be to some extent hindered. Even so, some researchers have devoted their investigations to the production of polyurethane foams prepared from oxypropylated cork powder [43].

Vegetable Oils as Polyol Sources

In search of the compliance of the sustainable development standards, vegetable oils have drawn the attention in the last decades as feasible alternatives for the partial or total replacement of unsustainable raw materials to produce cost-effective and renewable urethane polymers [44]. The excellent properties of such natural oils, including their abundant availability, non-toxicity, sustainability, easy handling and structural versatility, are the driving force of their consideration as polyol sources [45].

Vegetable oils are mainly composed of triacylglycerides, holding three aliphatic fatty acid chains composed of 12–22 carbon atoms and a double-bond functionality ranging from 0 to 3 per chain typically located at the 9th, 12th or 15th C-atom [45, 46]. Each vegetable oil exhibits a characteristic fatty acid profile, which might even vary within the same type of oil, depending on the plant variety and provenance [47].

The improved hydrolytic and thermal resistances due to the hydrophobic character of vegetable oils are remarkable qualifications. Nevertheless, what really makes natural oils such a reasonable chemical platform for bio-polyurethane development is the presence of amenable unsaturations and ester groups, providing them with exceptional chemical versatility [48].

Even though the production of appropriately functional polyols from vegetable oils has been reported through a series of chemical modifications, as will be further discussed, some studies propose the production of “green” polyurethane macromolecules by direct utilization of a limited group of natural oils containing hydroxylated fatty acids, essentially castor and lesquerella oils. In this way, Ferreira et al. [49] produced bioadhesives for tissue engineering by direct modification of castor oil with IPDI via prepolymer method, in contrast with the straight-through process applied by Raghunanan et al. [50] when preparing elastomeric adhesives from castor oil and HMDI. Moreover, Hablot et al. [51], Abdolhosseini and Givi [52] and Zhang et al. [53] applied the one-shot formulation procedure to synthesize elastomeric polyurethanes. Moreover, Sharma et al. [54] reported the systematic substitution of polypropylene glycol by castor oil for the production of polyurethane foams, proving the still need for only a partial replacement of petro-based polyols, given the loss in their properties when considering castor oil as the only hydroxyl group source. This fact has also been corroborated by Septevani et al. [55] who gradually substituted polyether polyol by a commercial polyester polyol derived from palm kernel oil, obtaining an appropriate foam behaviour of the ensuing polyurethane until a critical degree of substitution of 30%, above which the properties of the foam became poorer.

In the following sections, the major synthetic modifications affecting the functional active sites of not only vegetable oils but also fatty acids (C–C double bonds and ester groups) will be described.

Epoxidation

The formation of oxirane groups appears as one of the most widely employed double bond modifications for the production of polyols from vegetable oils. Epoxidation may be performed by using either prepared or in situ generated peracids (arising from the reaction between acids and hydrogen peroxide) and sulfuric acid as catalyst, followed by an additional ring-opening process (Fig. 3a). This hydroxylation step, essentially consisting of a nucleophilic attack, is the controversial and most engaging point in the polyol production, since an appropriate selection of nucleophile allows for the optimization of important factors such as hydroxyl functionality, the appearance of dangling chains and availability of the hydroxyl groups [56].

Fig. 3

Double bonds modifications: a epoxidation-hydroxylation; b hydroformylation; c ozonolysis; d air oxidation; e thiol-ene coupling

In this respect, the in situ oxirane ring-opening may be performed under acid conditions with water as the reagent, obtaining two secondary hydroxyl groups per double bond. Thus, Narute and Palanisamy [57] applied the epoxidation and in situ ring-opening with water on cottonseed oil aimed to synthesize bio-sourced polyurethane coatings with an optimum NCO:OH molar ratio of 1.8. However, the nucleophilic attack in this process can be conducted not only by water but also by acetic acid, as shown by the study of Pechar et al. [58], so that the polyol functionality halves. More recently, Mekewi et al. [59] simulated the in situ procedure by blending the epoxidized vegetable oil with deionized water and p-toluene sulfonic acid, achieving afterwards a partial replacement of the petrochemical polyols (50%) to produce ink plasticizer polyurethanes.

As a consequence of the intrinsic constraints associated to batch production, such as the unavoidable long reaction times, side reactions and the inherent start-up and shutdown stages, Ji et al. developed a novel continuous microreaction method with improved surface to volume ratio, thus enhancing the mass and energy transfer during epoxidation and subsequent nucleophilic attack with water in order to produce polyurethane rigid foams [60, 61]. In summary, these authors optimized the polyol production in two steps, first analyzing the epoxidation conditions (75 °C, 3% H₂SO₄, H₂O₂/formic acid/C=C of 8:8:1) leading to an optimal epoxy number of 7.3 in only 6.7 min of residence time [60], and afterwards, the hydroxylation step (75 °C, 10% H₂SO₄, 13 min) producing soy-polyols with more uniform molecular weights, characterized by a greater hydroxyl value [61], evincing the remarkable advantages of the microflow technology over the polyol production in batch.

On the other hand, when considering acids as the hydroxylation agents, the polyol functionality might be improved, like for instance in the lipase-catalyzed hydroxylation of epoxidized soybean oil with lactic acid performed by Miao et al. [62] or in the application of castor oil fatty acids, avoiding the utilization of either solvent or catalyst at the expense of increasing the temperature (130–190 °C) [63,64,65]. However, in some instances, such as in adhesive formulations that require gelation to ensure a suitable and safe adhesive application, a lower hydroxyl value with secondary functional groups may be required [66]. Moreover, inorganic acids such as hydrochloric o hydrobromic acids can also be used, albeit the utilization of a polar organic solvent may be essential to overcome their incompatibility with vegetable oils [67].

Furthermore, alcoholysis of the oxirane ring with monofunctional alcohols in the presence of an acid catalyst provides natural polyols with secondary OH groups [68]. In addition, given that the unsaturation density and location in the fatty acid chains are the significant variables in natural oils, Zlatanić et al. [47] analyzed the influence of hydroxyl functionality (from 3 up to 5.2) and dangling chain lengths of polyols manufactured from different oils on the resulting polyurethane, reporting the strong proportional relation between the glass transition temperature and the density of crosslinking with polyol functionality, while the impact of the dangling chains was negligible.

On the other hand, the conversion with polyfunctional polyols enables the simultaneous incorporation of secondary and primary hydroxyl groups into the C=C linkage. In this way, investigations dealing with diethylene glycol and 1,2- and 1,3-propanediol as hydroxylation agents can be found [69, 70]. Datta and Głowińska [71,72,73] employed propanediols derived from glycerol resulting from the biodiesel production and corn sugar fermentation processes, while Ventakesh and Viswanathan [74, 75] reported the application of castor oil-based fatty diols as the hydroxylation agent to prepare high molecular polyols from a series of vegetable oils, i.e. sesame, peanut neem, jatropha and mahua oils. Furthermore, Kong et al. [76] proposed an inexpensive combination of hydroxylation and transesterification with 1,2- and 1,3-propanediol in order to increase the polyol functionality and to reduce the molecular weight and viscosity of the resulting polyurethanes, increasing their resistance to hydrolysis and alkali attack. The polyol obtained from the combination of those modifications succeeded in such a way that it was quickly marketed by the name of Liprol™ (Meadow Polymers, Consolidated Biofuels Ltd., Canada), which would be further used in conjunction with cellulose nanocrystals to prepare polyurethane nanocomposites [77]. Another pioneering study is that conducted by Norhisham et al. [78], which addresses the production of biodiesel fuel-based polyols (fatty acid methyl ester) combined with palm olein polyol to synthesize elastomeric polyurethanes with greater natural content and an appropriate pressure-sensitive adhesive response, using boron trifluoride diethyl ether complex (BF₃) as the alcoholysis catalyst.

Catalytic hydrogenation is reckoned as another oxirane ring-opening reaction leading to secondary alcohols due to the incorporation of both hydrogen atoms into the double bond with the aid of a Raney nickel catalyst (Al-Ni) at high pressures [68]. According to Petrović et al. [79], this method shows certain flexibility, since the ultimate hydroxyl value of the hydrogenated polyol can be tuned by carefully controlling the reaction times, what inevitably influences the properties of the resulting soybean oil-derived polyurethane, being possible to obtain from glass to rubber systems. Conversely, Monteavaro et al. [80] modified the polyol functionality through different in situ hydrogenation times on epoxidized soybean oil as well, though the -OH functionality range (1.9–3.2) did not exceed the resulting from the Petrovic et al. study (2.7–4.4). Furthermore, in former investigations, Petrovic et al. [67] assessed the influence of the hydrogenation procedure on the hydroxyl functionality, so that epoxidized soybean oil was hydrogenated by using hydrochloric and hydrobromid acids, methanolysis and catalytic hydrogenation, demonstrating the improved functionality of halogen bearing polyols (up to 4.1), whereas the only liquid polyol was the methoxylated one at room conditions.

Finally, an alternative to the hydroxylation through a nucleophilic attack comprises ring-opening oligomerization and subsequent reduction of ester groups. Thus, Lligadas et al. [82] reported the epoxidation of methyl oleate and further oligomerization catalyzed by fluoroantimonic acid (SbHF₆), followed by a partial reduction of the carboxylate functional groups with lithium aluminum hydride (LiAlH₄) at room temperature, aimed to incorporate primary hydroxyl groups into the polyether polyol chemical structure (Fig. 4). A similar reduction process was conducted by Zhang et al. [64, 83] on epoxidized (soybean and linseed) and raw (castor) oils, but at low temperatures (0 °C), thus obtaining hydroxyl values at least threefold higher than those reported by Lligadas et al.

Fig. 4

Oligomerization of fatty acids followed by carboxylate groups reduction [81]

Hydroformylation

Hydroformylation falls into the category of modifications providing primary alcohols from natural oils or fatty acids, without increasing their functionality, as only one OH group can be incorporated per double bond. Therefore, first, an intermediate aldehyde is typically produced using an inlet flow of synthesis gas (CO/H₂) and a cobalt or rhodium catalyst, then conducting the followed reduction with H₂ and a Raney nickel catalyst (Fig. 3b). The choice of the catalyst is particularly important as it will directly affect the reaction yield (Co-67% vs. Rh-95%). At the expense of a reduction in the yield of the reaction, the utilization of cobalt-based catalysts entails some benefits, essentially their lower prices and the possibility to act as hydrogenation catalysts as well, though under more severe conditions [68, 84]. Furthermore, the type of catalyst considered in the production of the natural polyol may also influence the response of the resulting polyurethane, as Guo et al. reported when producing hydroformylated soybean oil [85], obtaining bio-based polyurethanes whose behaviour ranged from rigid plastic to hard rubber polyurethanes when using rhodium and cobalt catalysts, respectively.

Ozonolysis and Hydrogenation

This oxidation procedure emerges as an alternative to previous modifications, where the synthesized polyols exhibit dangling chains with their respective plasticizing effect [56] since ozonolysis allows for the synthesis of polyols with primary terminal hydroxyl groups, provided that vegetable oils solely comprise unsaturated fatty acids. This greater OH availability may be translated into faster condensation reactions with isocyanates thanks to the lower steric hindrance, despite its functionality limitation of a maximum of 3 hydroxyl groups per vegetable oil molecule, and the production of alcohols of low molecular weights [86] typically removed due to their deleterious effect on the final polyurethane, diminishing its microphase separation and the associated beneficial properties [87].

Ozonolysis comprises the formation of ozonide when treating double bond with ozone (O₃), which subsequently decomposes into aldehyde and carboxylic functional groups, giving rise to the C=C cleavage. Afterwards, the already known hydrogenation step with Raney nickel catalyst is performed to reduce aldehyde groups into primary alcohols (Fig. 3c) [68, 84].

Nevertheless, some of the drawbacks of the ozonolysis-hydrogenation of double bonds are the extreme pressures and temperatures, and the requirement for toxic solvents in the final reduction [42], inasmuch as initial ozonolysis was reported to be conducted in deionized water at 0 °C and the subsequent hydroxylation in tetrahydrofuran (THF) under 130 °C and 350 psi of hydrogen pressure [88]. Thereupon, Kong et al. [89] devoted one study to the enhancement of the previous protocol via the replacement of the traditional hazardous THF by a more environmentally friendly ethyl acetate during the whole process, using zinc as a reagent for the reduction of the ozonide groups to aldehydes at room temperature. With all these improvements, along with the reduction of temperature and pressure during hydrogenation (70 °C and 100 psi), an increase of the hydroxyl number from 152.4 up to 235.2 mg KOH/g was obtained, much closer to the theoretical 251 mg OH/g. On the other hand, the efficiency of canola oil ozonolysis was even higher when performing the ozonolysis at lower temperatures (from − 30 to − 40 °C) in a blend of methylene chloride/methanol, followed by a reduction with sodium borohydride (NaBH₄) at 10–15 °C [90]. Moreover, another variant worthwhile to mention is the one-step ozonolysis in the presence of NaOH, CaCO₃ or H₂SO₄ catalysts and polyols that will interact with the ozonide functional groups to produce polyester polyols (Fig. 5) [91].

Fig. 5

One-step ozonolysis

Oxidation with Air

Similarly, oxidation with oxygen (O₂) can provide plant oil-derived polyols by the first production of hydroperoxide (–OOH) through a UV-radiation process with thiamine pyrophosphate in dichloromethane, followed by the reduction with NaBH₄ in methanol at 0 °C and a catalytic hydrogenation step in ethyl acetate (Fig. 3d). However, the requirement for reaction times in the order of days and the poorer oxidation control compared with other modifications, such as epoxidation, have restricted the application of air-oxidation to synthesize bio-sourced polyurethanes [68, 86].

Thiol-ene Coupling

The last modification regarding fatty acid chain unsaturation is the so-known thiol-ene coupling. This process, regarded as click chemistry and based on the addition of thiol linkages (HO-R-SH, mainly 2-mercaptoethanol) onto C=C double bonds, leads to polyols with primary hydroxyl groups through an eco-friendly and highly efficient radical mechanism (Fig. 3e). This procedure may be initiated by either UV-radiation [92] or thermal induction [93].

The appeal of this reaction comes from the possibility of producing bio-based polyols with a wide range of functionality through a simple, solventless, efficient and cost-competitive procedure under air conditions [87], despite presenting some disadvantages such as the presence of dangling chains and the production of some by-products [56]. However, as most of the products resulting from the side reaction have been proved to be hydroxyl-bearing compounds, they could still participate in the polycondensation reaction with reactive isocyanates. Desroches et al. [92] addressed in their study the optimization of the thiol-ene coupling conditions (intensity of radiation, solvent and double bond contents) for the photoaddition of 2-mercaptoethanol onto oleic acid, to be further applied on rapeseed oil, thus reducing the reaction time, but still obtaining byproducts.

Furthermore, it is possible to obtain polyols with terminal primary hydroxyl groups if the photochemical grafting of thiols is combined with allyl esterification, as Lligadas et al. reported when preparing diols via thiol-ene coupling after esterification of oleic and 10-undecenoic acids with allyl alcohol [94], and even controlling the chain length through a step-growth photopolymerization using a dithiol followed by a phototermination with 2-mercaptoethanol producing polyester polyols with a tailorable molecular weight [95]. The versatility of thiol-ene click reaction was also highlighted by the investigations of Alagi et al., who succeeded in both controlling the soybean-based polyol functionality [96] and accomplishing the quantitative conversion to hydroxyl of soybean and castor oils unsaturations [97] by sensibly adjusting the reaction time and temperature, along with reagents’ concentrations. Moreover, the preparation of multifunctional polyols via thiol-ene coupling incorporating silane, fluorine and ethylene oxide [98] or acrylate [99] functional groups has been recently reported, aimed to improve either the coating properties or the UV-curing speed, respectively.

Transamidification and (Trans)esterification

Although the modification of fatty acid chains has been focused on the carbon to carbon double bonds, the production of bio-based polyols via ester functional group transformation is also attainable, by means of transamidation and (trans)esterification processes.

Amidification consists of the reaction of vegetable oils with amines and a catalyst at high temperatures (100–120 °C) leading to amides and alcohols, although diethanolamine is typically regarded as amidification agent of plant oils, alcoholamides are commonly obtained together with glycerol (Fig. 6b) [68]. Moreover, if triethanolamine was considered as the reagent, a transesterification process would occur, leading to poly(ester urethane) instead of the previous poly(urethane amide) [100]. In addition, a wide array of catalysts has been reported, ranging from metal oxides [101] to different salts, like stannous octoate [102] or zinc acetate [103].

Fig. 6

Ester group modifications: a transesterification; b transamidification; c direct esterification

Especially noteworthy is the progress achieved by Patil et al. [104], succeeding in the rise of polyurethane coatings natural content up to 65% with fairly competitive behaviour against the petro-based counterparts, by means of a further esterification of the cottonseed oil-based diethanolamide with bio-sourced difunctional carboxylic acids, inter alia succinic, sebacic, tartaric, maleic and azelaic acids, thus producing polyesteramide polyols as polyurethane precursors.

Conversely, transesterification of vegetable oils comprises the reaction with multifunctional polyols, mainly glycerol, and catalyzed by enzymes and organic and inorganic catalysts at high temperatures (170–200 °C) (Fig. 6a) [56, 105]. Nevertheless, when considering heterogeneous catalysts, like lead oxide (PbO), greater temperatures are required, achieving values of up to 240 °C, according to the resinification of Nahar seed oil described by Karak et al. [106,107,108] for the development of monoglycerides to synthesize polyurethane resins. Unlike amidification, transesterification is a thermodynamically balanced reaction capable of modifying plant oil in a single step, which places it among the most appealing modifications [87]. However, this reaction provides a mixture of the residual glycerol with mono-, di- and triglycerides, so that the maximum functionality is also limited, restricting the OH-source for polyurethane synthesis to mono- and diglyceride, along with the remaining transesterification agent. Moreover, the presence of dangling chains could only be advisable when flexible polyurethanes are intended to be obtained.

Due to the low apparent thermal resistance of glycerol, some studies deal with the utilization of other transesterification agents, such as pentaerythritol [109] or triethanolamine [103], as above indicated. Furthermore, a more specific reaction can be conducted when dealing with non-hydroxyl-bearing plant oils, viz. oils interesterification, as reported by Saravari and Praditvatanakit [110] for the production of urethane alkyd from interesterified jatropha/castor oils.

Direct carboxylic acid esterification arises as an alternative to transesterification, whether long molecular chain polyols are the primary target, obtaining water as a byproduct instead (Fig. 6c) [68]. Therefore, the so-called fusion of lauric [111] and ricinoleic [112] acids with ethylene glycol, as well as the production of polyester polyols from direct esterification of dicarboxylic acids with castor oil [113] have been reported to produce polyester polyurethane with adhesion properties.

However, these three individual chemical routes cannot be necessarily applied independently, as demonstrated not only by the transesterification with methanol and subsequent amidification of the resulting methyl ester [114,115,116] but also several-step modification of vegetable oils via transesterification-direct esterification [117] or transamidification-direct esterification [104] in order to produce polyester or poly(ester amide) diols, respectively.

Other Modifications

Although the modifications above detailed are the most widely applied on vegetable oils and fatty acids, some other transformations have been spread to a lesser extent to produce polyurethane precursors, including dimerization and subsequent reduction of carboxylic groups to polyols [118], self-metathesis coupling producing a diol to be used as an unsaturated chain extender [119] or even cross-metathesis prior to other carbon-to-carbon double bonds modifications aimed to produced terminal hydroxyl groups [120].

Waste Products as Polyurethane Feedstocks

The ever-increasing worldwide population, which has increased in the last 60 years in 136.9 million people [121], implies a huge consumption of a great variety of end-use products and its corresponding agricultural and household disposal. Consequently, recycling and reutilization of waste materials have drawn the attention in the research field in pursuit of “greener” processes and products.

In this sense, residual pine cone [122], an agricultural waste, has been suitably pretreated with Fenton’s reagent to be further crosslinked with hexamethylene diisocyanate in order to be applied as an effective absorbent for organic water-pollutants. In addition, cardanol, a phenolic oil obtained as a byproduct of the cashew industry, enables the polyurethane production from a different perspective, despite using the above described synthetic routes, like epoxidation of double bonds and further oxirane ring-opening, as reported by Suresh [123] who applied afterwards an additional saponification step to increase the hydroxyl value of the oxidized cardanol. By contrast, Balgude et al. [124], apart from directly using a commercial-grade epoxidized cardanol, considered only the epoxy ring-opening with multifunctional natural acids (tartaric, citric and adipic) to produce hydrophilically modified cardanol as the starting polyol for waterborne polyurethane coatings.

In addition, the massive production and consumption of poly(ethylene terephthalate) (PET) has encouraged the development of new recycling ways to mitigate its environmental impact. Among other mechanisms, the depolymerization of PET-soft drink bottles via liquefaction with propylene [125], polyethylene [126] or triethylene [127, 128] glycols has been reported by Cakić et al. for the preparation of polyurethane dispersions. In contrast with the zinc acetate-catalyzed liquefaction previously described by Cakic et al., Scremin et al. [129] devised for the first time the employment of sodium metasilicate as a more accessible and cheaper alternative to the former alcoholysis catalyst, leading to similar conversion yields. Additional to the depolymerization step, a transesterification with castor oil has also been conducted, incorporating silica nanofillers [127, 128, 130]. Moreover, Zhou et al. [131] recently verified the novel incorporation of recycled PET into both, soft and hard segment domains in waterborne polyurethanes, owing to the utilization of glycolyzed oligoesters as polyols and chain extenders in the PU synthesis.

Crude glycerol also appears as a feasible choice given its vast production as a secondary product in the biodiesel industry [56], so that it has been successfully converted into natural polyols by means of direct esterification with fatty acids to prepare polyurethane adhesives for bonding wood [132, 133], similarly to the transesterification with diethanolamine of tall oil obtained from Kraft pulping [100].

Finally, carbon dioxide, an inexpensive and abundant greenhouse gas, can offer a new synthetic route in the manufacturing of polyurethanes [134], contributing to the alleviation of environmental pollution [135]. The synthesis of the CO₂-based polyols consists of the fixation of CO₂ with the aid of oxirane groups-bearing materials, using a metal catalyst, leading to polycarbonate polyols (Fig. 7), which have served to prepare footwear polyurethane adhesives [136] or mussel biomimetic pH-sensitive hydrogels [137].

Fig. 7

Oxirane ring carbonatation

Synthesis of More Sustainable Isocyanates

All the renewable alternatives so far reviewed are founded on the development of “greener” polyurethanes by generating bio-based polyols, without paying attention to the other PU-major component, the isocyanates, which come from petroleum resources [42].

Among other synthesis mechanisms, the conventional Curtius and Hoffman rearrangements, which consist of the rearrangements of acyl azide or amide groups, respectively, are typically employed to produce isocyanates at the laboratory scale. Moreover, the amine phosgenation (Hetschel) is the main isocyanate production commercially applied, in spite of the fact that it requires highly toxic phosgene (Fig. 8) [4, 138].

Fig. 8

Synthesis of isocyanates

However, the application of hazardous phosgene and the associated environmental awareness, have given rise to the seek for more eco-friendly alternatives, such as substitution of phosgene by other equivalents, like triphosgene to produce isocyanates at large scales [139], or dehydration of carbamate resulting from the carbonylation of amines with CO₂ [140], although none of them has been yet industrially established [141].

Even more interestingly, the increasing renewability of polyurethanes has been achieved by synthesizing isocyanates from natural feedstocks, such as vegetable oils and fatty acids. Thus, Çaylı and Küsefoğlu [142] conducted the allylic bromination of soybean oil, followed by its reaction with AgNCO at room temperature in THF, leading to NCO-terminated soybean oil, whereas Hojabri et al. [141], in an effort to reduce the steric hindrance of the present dangling chains, proposed a Curtius rearrangement of diacids with sodium azide obtaining a linear aliphatic diisocyanate (Fig. 9a). This process was later improved obtaining even longer unsaturated diacids when using a 2nd generation Grubbs catalyst, avoiding the occurrence of explosive intermediate azelaic acid, to further synthesize the bio-based diisocyanate (Fig. 9b) [44]. In addition, More et al. [4] efficiently synthesized highly pure oil-based isocyanates through a several-step protocol based on simple organic reactions in which diacyl hydrazide appears as an intermediate product.

Fig. 9

Soybean oil-based diisocyanates synthesis reported by Hojabri et al. a [141] and b [44]

In the last decade, bio-sourced diisocyanates, mainly coming from aminoacids (l-lysine diisocyanate) and fatty acids (dimeryldiisocyanate), have been the focus of several investigations, yielding renewable polyurethanes whose response appears to lack of toxicity, according to the in vivo cytotoxicity analysis results reported by Calvo-Correas et al. [13].

Development of new synthetic pathways

In accordance with the Green Chemistry tenets, the application of synthesis protocols involving “safer solvents and reaction conditions” [18] is a further step to a sustainable production of PUs, as may be inferred from the recent investigation of Boulauche et al. [143], when conducting a solvent-free cationic polymerization, by dint of the application of a heterogeneous ecofriendly catalyst, namely maghnite-H⁺. As a consequence, the production of waterborne, radiation-curable and non-isocyanate polyurethanes have emerged in response to the ever-increasing environmental awareness. The following sections briefly describe these synthetic routes.

Waterborne Polyurethanes

Water-based polyurethane dispersions, also known as waterborne polyurethanes (WPU), allow for the replacement of the traditional solvent-borne polyurethanes, thus reducing the environmental impact, as revealed by the comprehensive approach of the life cycle assessment conducted by Maciel et al. [144], besides exhibiting some beneficial advantages, inter alia versatile structure-properties relationship and good processability and elasticity [145, 146].

Waterborne polyurethanes constitute the group of polyurethane systems composed of a dispersion of binary colloidal particles in an aqueous matrix, so that, as a consequence of their characteristic hydrophobicity, there is the need to insert water-compatible stabilizing groups (emulsifiers) into the polyurethane matrix. These emulsifiers consist of hydrophilic and amphiphilic compounds which can be classified into internal and external emulsifiers [145, 147]. Although a wide range of internal emulsifiers, either ionic (cationic, anionic or even zwitterionic) or non-ionic, could be considered, including bis(hydroxymethyl)propionic acid [148] or 2,2-dimethylbutanoic acid [149] among others, dimethylol propionic acid (DMPA) has been the most extensively used [150,151,152,153], even finding in the literature simultaneous application of several emulsifiers in order to synthesize finer WPU dispersions [154].

Furthermore, these polyurethane dispersions may be prepared by different procedures, being the prepolymer [155, 156] and acetone [157,158,159] techniques the most extensively reported. Briefly, in the prepolymer method, the hydrophilic NCO-terminated prepolymer is synthesized in a first step from the emulsifier and the conventional polyurethane precursors (polyol, diisocyanate and diol chain extender). Then the carboxylic groups of the ionic emulsifier are neutralized with amines (typically triethylamine), followed by the prepolymer dispersion in water. The last chain extension with diamines terminates the preparation of waterborne polyurethanes, as shown in Fig. 10. Conversely, the acetone process follows the same steps but performing the neutralization and further chain extension in acetone, or other water-miscible organic solvents, to ensure homogeneous conditions. This low boiling point solvent is removed through vacuum distillation after dispersion in water (Fig. 10).

Fig. 10

Waterborne polyurethane synthesis via prepolymer (solid line) and acetone (dashed line) processes

Moreover, the significant influence of the synthetic procedure on the waterborne polyurethane properties has been corroborated by Pérez-Limiñana et al. [160], who demonstrated the narrower particle size distribution provided by the acetone process over the prepolymer method, due to the promotion of the prepolymer dispersion in water resulting from the lower prepolymer viscosity in the presence of an organic solvent. However, the properties of the final WPU, such as phase separation, thermal and mechanical resistance or the particle size, have been proved to be deeply dependent on other reaction factors, such as the ionic content [161] or chain extender chemical structure [162].

The significant impact of the design of such sustainable synthetic route becomes noticeable in studies as that conducted by Li et al. [14], revealing the preparation of polyurethane systems with around a 90% of renewable content, owing to the combination of WPU synthesis procedure with the utilization of renewable resources, namely isosorbide polyol and fatty acid-based isocyanate. Additionally, Zhou et al. [131] and Cakić et al. [130], for their part, properly incorporated recycled PET bottles in the polyurethane structure via glycolysis, whereas Saetung et al. [146] used polyols based on natural rubber and rubber seed oil acting as the starting materials for renewable WPU for application in the footwear industry.

Waterborne polyurethanes have been mainly intended for eco-friendly adhesive production, even though other purposes such as gravure printing ink [151, 163], thermally responsive elastomers [164] or biomedical coatings [165] have been reported. Notwithstanding, regardless of the application field to which these systems can be meant, there are still some shortcomings to be remedied, associated with the film stability in terms of mechanical, thermal and chemical resistance, or the lack of biocompatibility and antibacterial activity. To that end, in situ hybridization with silica [127] and sulfonic [166] amine chain extenders, or diols more sensitive to hydrolysis linkages [150] like, for instance, those holding silver nanoparticles [167] or long aliphatic branches [151, 163], enable the customization of the resulting WPUs. Indeed, as Tsen et al. pointed out [153], the in situ hybridization with 2,2,3,3,4,4,4-heptafluorobutynic acid 2,2-bis-hydroxymethyl-butyl ester (HFBA) to WPUs confers them impressive thromboresistance values, resulting in competitive fluorinated polyurethane dispersions to be applied as biomaterials in living organisms. Following such in situ copolymerization approach, Chen et al. [168] prepared novel high viscosity waterborne polyurethane-cellulose nanofibril composites for the synthesis of direct three-dimensional printing ink, holding good shear-thinning properties, what enabled the production of biomedical engineering scaffolds with improved mechanical behaviours and a degradability threefold faster than a conventional PU. Conversely, a simpler ex situ incorporation of modifiers including silica nanoparticles [169, 170], cellulose nanocrystals [155], ferric ions [171] and thickeners [172] can be successfully carried out.

More interestingly, the copolymerization with acrylic monomers can provide the desired properties for waterborne polyurethanes, while simultaneously producing a controlled crosslinking process and replacing the organic solvent required to adjust the viscosity of the medium. Even though some investigations have dealt with the addition of acrylic polymer chains by radical polymerization of acrylate monomers without chemical interaction between urethane and acrylate molecules [173], grafting of hydroxyl bearing acrylate monomers (hydroxyethyl acrylate, hydroxyethyl methacrylate, bisphenol-A-glycidyl dimethyl acrylate, etc.) as anchoring sites covalently embed into the PU backbone leads to stable WPU/acrylic hybrids, whose polymerization step is typically accomplished with UV-radiation and a photosensitive compound [174] or in the presence of potassium persulfate (KPS) as radical initiator [175]. Besides, Alvarez et al. [176] have recently proposed a new radical polymerization methodology based on a redox initiating blend of ascorbic acid and hydrogen peroxide, allowing for conducting the polymerization at 30 °C.

Radiation Curable Polyurethanes

Polymerization of polyurethanes with the aid of ultraviolet (UV) radiation has recently drawn the attention due to its remarkable advantages, namely process simplicity, mild reaction conditions, fast and efficient drying process, along with the absence of volatile organic solvent in an easily controlled curing process. The synthesis of photo-crosslinkable polyurethanes requires the usage of UV-sensitive monomers, mainly acrylates, and a photo-initiator, together with the traditional polyurethane building blocks, leading to the production of polyurethane acrylate systems. Therefore, some hydroxyl-containing monomers, such as 2-hydroxyethyl acrylate (HEA), 2-hydroxyethyl methacrylate (HEMA) or 2-hydroxypropyl acrylate (HPA) (Fig. 11), are typically considered to facilitate their grafting via polycondensation between their hydroxyl groups with reactive diisocyanates, so that the double bonds confer their UV-light activity. The addition of these reactive diluents is also of great importance, since they confer these systems a suitable viscosity, apart from facilitating the control of the polymerization process and improving the ultimate film properties [177].

Fig. 11

Chemical structure of the most common UV sensitive acrylate monomers: a HEA; b HEMA; c HPA

This technique is particularly useful in biocompatible adhesives and tissue engineering, where a quick curing step is required, despite the need for oxygen inhibition in order to avoid likely side reactions, quite achievable by modifying the radiation intensity and initiator concentration [178]. Thus, Liu et al. [165] intended to avoid attachment of microorganisms to WPU coatings by embedding lysozyme into the polyurethane backbone via photopolymerization, while Abdalla et al. [179] effectively prepared homogeneous biocompatible adhesives based on polycaprolactone and 2-isocyantoethylmethacrylate in just 60 s. Such UV-light susceptible isocyanates have also been applied in other studies [180, 181] pointing out that UV-induced radically polymerizable functional groups can be found not only in additional monomers but also in the isocyanate source.

As with the production of polyurethane dispersions in water, UV polymerization technique may be combined with other strategies towards sustainable production, so that it can be supplemented with the synthesis of natural polyols via fatty acid esterification [182], thiol-ene coupling of vegetable oils [99] or liquefaction of bagasse [183]. In addition, the combination of this photo-crosslinkable technique with waterborne systems has also been reported by Wei et al. [184], and Bakhshandeh et al. [185] who produced UV-curable polyurethane dispersions from renewable castor oil and hydroxyl-terminated polydimethylsiloxane, respectively, to be applied as coatings after water evaporation. More remarkable is the dual curing polymerization proposed by Hu et al. [186] when preparing bio-based coatings from castor oil-based polyurethane/acrylate and epoxidized cardanol glycidyl ether, so as to reduce the shrinkage previously observed during radical polymerization. Not long after, Hu et al. [187] considered the preparation of cardanyl acrylate from cardanol and acryloyl chloride, in order to be considered as reactive diluent for the preparation of photo-crosslinkable polyurethanes, in an effort to further improve in more than a 10% both the renewability and volumetric shrinkage of the resulting UV-curable polymers. Furthermore, the additional utilization of bio-based polyols has been reported as well, such as dextran [188] or rosin [189].

Non-isocyanate Polyurethanes

Although the vast majority of the research production has dealt with the sustainable polyurethane synthesis by using natural occurring building blocks, one sector of the research field has opted for taking a further step removing the hazardous isocyanate from the polyurethane production, thus meeting the current environmental regulations aimed to reduce the exposure to isocyanates [190], in such a way that polyurethanes thereby produced are known as non-isocyanate polyurethanes (NIPU).

Despite the fact that different synthetic protocols can lead to NIPUs, as reviewed recently by Cornille et al. [191], exclusively the polyaddition between bicyclic carbonates and diamines, as shown in Fig. 12, has proved to prevent the utilization of toxic substances (phosgene, aziridines, azides, carboxamides, etc.) and the production of byproducts. Since the first patent on NIPU preparation published by Groszos and Drechsel [192], this synthesis protocol has turned into the most relevant and used synthetic pathway for the production of bio-based polyurethanes.

Fig. 12

Non-isocyanate polyurethane synthesis

The appearance of pendant hydroxyl groups through this aminolysis reaction, whose primary or secondary character heavily depends on the synthetic path followed (Fig. 12), is the reason why this group of polyurethanes is also known as polyhydroxyurethanes (PHU). The improved hydrophilicity entails stronger cohesive forces, apart from enhanced thermal and chemical resistance owing to the establishment of inter- and intra-hydrogen bonds, which, in turn, gives them the chance to act as reactive polyols in further chemical modifications.

The reaction mechanism of this process has been analyzed from different standpoints, so that, according to Tomita et al. [193], this reaction is controlled by differences in electron densities giving rise to the appearance or breaking of chemical linkages, while to Zabalov et al. [194] it is based on a single- or several-step procedure in which a second amine molecule acts as catalyst. Anyhow, it was disclosed the second-order character of this reaction [193], besides the fact that primary amines and polar solvents seem to promote the addition reaction [195].

The main disadvantage of this synthetic route is related to its low reaction rate at room conditions, given that a thermal activation may prompt side reactions [191]. Therefore, the solution that best addresses this issue is the employment of catalysts, since, while an increase in the number of members of cyclic carbonates has frequently proved to provoke a reactivity rise [196, 197], their synthesis requires to be conducted under well-defined conditions, along with the application of harmful materials, especially regarding the manufacture of bicyclic carbonates [56].

Moreover, although cyclic carbonates may be obtained through transesterification and transcarbonation with diols [198], the catalytic production of the most extensively utilized 5-membered cyclic carbonate via CO₂ fixation into oxirane groups seems to be a much more appealing method to pursue an improved renewable content, while mitigating the environmental impact of this greenhouse gas [199]. Hence, the synthesis of bicyclic carbonate from carbonation of bisphenol S-based diglycidyl ether, as described by Kim et al. [200], offers a yield of 74%, similar to the 76% reported by Cornille et al., whose polyfunctional cyclic carbonates conferred quantitative conversion with diamines in the presence of a thiourea catalyst, leading to non-isocyanate polyurethane with appropriate foam [201] and adhesion [202] performances. Moreover, Arvin et al. [203] recently prepared NIPUs from carbonate sucrose soyate synthesized through CO₂ fixation on epoxidized sucrose soyate under supercritical conditions, optimizing the solvent resistances of the ensuing polymer coatings when using a dual Lewis acid/Lewis base pair catalyst and conducting a 1 week curing procedure at room temperature. On account of such a high pressure requirement associated to the CO₂ fixation, Wang et al. [204] promoted the production of NIPUs by conducting a sustainable synthesis protocol under milder and safer reaction conditions for the preparation of those cyclic carbonates comprising a first O-chloro alcohol resin synthesis from phthalic anhydride and pentaerythritol, and its subsequent reaction with sodium bicarbonate, leading to polyfunctional cyclic carbonates.

Even more sustainable production of NIPUs deals with obtaining bio-based cyclocarbonates by means of a lipase B-catalyzed esterification of Sapium sebiferum oil-derived dimer acid with glycerol carbonate [205], or applying an easily scalable catalytic procedure from levulinic acid biomass-derived diphenolic acid, leading to renewable WPU coatings [206]. On the other hand, Thébault et al. [207] reported the preparation of PHUs with aminated and carbonated mimosa tannin extract, achieving a bio-sourced content above 70%. By contrast, Tryznowski et al. [196] attained a 79% yield in the synthesis of five-membered cyclic carbonate from diglycerol to further produce non-isocyanate polyurethanes via one-step protocol at high temperatures. Further on, the wettability of these NIPUs was subjected to a deeper investigation [208], revealing that the low hydrophobicity of polyhydroxyurethanes may have a detrimental effect on their performance under high moisture conditions. Besides, these systems typically exhibit reduced hardness and glass transition temperatures, giving rise to inefficient chain mobility [205]. Although Cornille et al. [201] were capable of increasing the movement of the molecular chains with the utilization of a blowing agent, that methodology applies only for foams.

Therefore, other methodologies, essentially hybridization, have been considered. Thus, in an attempt to replace traditionally isocyanate-based polyurethane adhesives, Leitsch et al. [209] claimed to report the pioneering production of PU/PHU hybrids first conducting a conventional NCO-OH condensation, followed by chain extension with diamines, thanks to the previously grafted 1,2-carbonate glycerol. Although, according to Leitsch et al., the presence of free isocyanates is avoided during the adhesive application, they still participate in the early stages of the synthesis. In this sense, the preparation of PHU hybrids with epoxy [210, 211], polyhedral oligomeric silsesquioxane [212] and amide [213] groups appears to provide a safer way to tailor the PHU properties while preventing the use of isocyanates. More interestingly, Wang et al. [214] completely averted pendant hydroxyl groups in NIPU’s chemical structure by the reaction between diamines and monocyclic carbonates, and further esterification with dicarboxylic acids, as shown in Fig. 13.

Fig. 13

Non-isocyanate poly(ester urethane) synthesis [214]

Bio-polyurethanes in the Adhesion Field

Fundamentals and General Concepts

Since their inception, polyurethanes have become widespread in the adhesive field, being applied on an extensive array of surfaces (glass, wood, plastics, ceramics, etc.). Thus, starting with the urethane prepolymers during the fifties, followed by the waterborne polyurethanes patented by DuPont [215] and ending with the most recent pressure sensitive and hot melt adhesion techniques, polyurethane adhesives have carved out a well-established market niche.

Adhesion can be defined as the phenomenon of joining two substrates, so that it is not an exclusive property of adhesives but, in general terms, also of coatings, paints, varnishes, etc., in other words, whenever two solids are put together [216]. Therefore, adhesion is the result of a several-step process in which the adhesive flows and wets the surface, establishing certain physicochemical interactions in the adhesive mass (cohesion) and with the substrate (adhesion), including Van der Waals and dipole forces, along with stronger hydrogen and covalent bonds. Hence, a suitable wettability, thus promoting an intimate adhesive-substrate contact, is an important factor to take into consideration, which can be achieved whether the intermolecular forces of the adhesive are weaker than the corresponding interaction with the adhesion surface, meaning surface energy lower than the corresponding to the adhesion substrate [2].

Additionally, the performance of an adhesive is not only determined by its properties, such as rheological behaviour or surface free energy, but also by the adhesion surface characteristics (pH, functional groups, roughness, presence of debris or extractives) and curing conditions (temperature, humidity, etc.). As a consequence, some investigations have revealed the relevant influence of parameters such as the relative humidity [217], artificial aging [218] and substrate modifications to improve the adhesive wettability [219] on the ultimate properties developed by the adhesive joints.

The already discussed heavy reliance on industrial production of crude oil, also concerning the adhesive field, has been the driving force for the removal of toxic volatile organic compounds (VOC) from adhesive formulations traditionally used as carrier fluids [220]. Nonetheless, the well-known efficient wettability, hydrogen bonding and ability to establish chemical interactions associated with polyurethanes place them in a favorable position to suitably satisfy the requirements for the production of bioadhesives. Additionally, bearing in mind the progress made in the manufacturing of bio-sourced polyurethanes, as described in preceding sections, these “greener” adhesives seem to be increasingly meeting the definition of “bio-based adhesive” coined by Pizzi [221], which states that “bio-based adhesives include those materials of natural, non-mineral, origin which can be used as such or after small modifications to reproduce the behaviour and performance of synthetic resins”.

Latest Advances in Bio-based Polyurethane Adhesives

As reviewed throughout the previous sections, the widely flexible composition of polyurethanes adhesives has made them such versatile materials that they can be applied in many areas, such as in automotive [222], footwear [173], woodworking [223], etc. In general, as a consequence of such extensive array of polyurethane adhesives, several classifications can be found on the basis of different criteria, whether physical state (solid or liquid), number of components to blend to promote the adhesion (single or two components), curing mechanism (thermosetting or thermoplastic) or carrier (solvent-borne, water-borne or solvent-free). These classifications meeting different approaches are summarized in Table 1.

Table 1 Polyurethane adhesives classifications [2]

Two-component polyurethanes represent those adhesives traditionally produced whose components are delivered in separate compartments given their commonly high reaction rate. In these systems, whether solvent- or water-based, the curing process is mainly occasioned by carrier release after urethane linkages formation, while solvent-free polyurethanes yield adhesive joints exclusively due to NCO-OH reaction. Conversely, one-component adhesives, when reactive, consist of NCO-terminated prepolymers, in which the reaction with proton donor groups on the substrate and/or with atmospheric humidity (moisture curable polyurethanes) is the driving force for curing, whereas those containing a carrier, like in two-component PUs, curing comprises the coalescence of precipitated particles owing the removal of the carrier fluid. Moreover, hot melt PU adhesives are solid typically melted, thus acquiring appropriate wettability, leading to the adhesive bond by cooling from the molten state, although on some occasions combined with moisture reactions (reactive hot melt adhesives) [2].

As reflected by extensive research production on moisture curable adhesives [224,225,226,227], such type of one component polyurethanes has been widely applied. Notwithstanding, as stated above, the main obstacle of the consequent curing step is the production of CO₂, which may diminish the adhesion performance as a result of the appearance of bubbles or pores. In this regard, some studies have avoided the carbon dioxide production by synthesizing silane-modified PU, releasing ethanol instead of CO₂ as a result of the curing reaction with environmental humidity [228], while Yuan et al. [229] reported the implementation of oxazolidines as latent curing agent, since their hydrolysis leads to alkanolamines as intermediate products, further reacting with free isocyanate groups (Fig. 14). The ketones, produced as byproducts, are easily volatilized. With this modification, these authors succeeded in raising the shear strength of single-component polyurethanes eight times, while attaining 20-fold greater adhesion forces via the addition of CaCO₃ as a filler at 23% of the content. Filler incorporation allowed them to prepare smoother adhesive surfaces with reduced pores content as a consequence of the reinforcing water absorption effect of CaCO₃ particles. Therefore, these authors successfully produced more environmentally-friendly polyurethane adhesives, in a broader sense than the one associated with the definition of “bio-based adhesives” abovementioned [221]. Indeed, as previously stated, according to the Green Chemistry principles [18], the “sustainability” of adhesives lies not only in their provenance, i.e. their chemical constituents, but also in all aspects associated to their synthesis and application, such as the reduction of the CO₂ release related to the adhesive end-use.

Fig. 14

Latent curing mechanism of oxazolidines [229]

Furthermore, the great versatility of the urethane polymer network has unleashed a myriad of investigations [112, 113, 133, 230, 231] dealing with judicious variations in the NCO:OH molar ratio in order to optimize the PU adhesion response. Even though some researchers achieved optimal properties at higher NCO:OH ratios [232, 233], most of them reported a generalized value of 1.3 to obtain more appropriate adhesion performance, giving rise to not only more interesting type of failure, i.e. substrate [113], but also to improved lap-shear strengths, as described by Cui et al. [132, 133] who developed PU adhesives with shear strengths of around 36 MPa, fairly competitive with the 29-37.7 MPa shown by commercial benchmarks based on petrochemical raw materials. Cui and coworkers succeeded in synthesizing fast-curing adhesives from crude-glycerol, whose curing times have proven to be below 1 h, even shorter than those accomplished by PU adhesives based on castor [112] or canola [220] oils.

On the other hand, the adjustment of adhesion properties of polyurethanes has also been addressed by changing other parameters, such as polyol hydroxyl value [133] or chemical structure of chain extenders [162]. Particularly significant is Somani’s investigation [112], which revealed the superior properties of aromatic isocyanates over their analogous aliphatic, leading to castor oil-based aromatic polyurethanes with lap-shear strengths around seven times greater. Overall, larger polyol functionalities promote greater crosslinking densities, resulting in improved glass transition temperature (T_g), chemical and hydrolysis resistance [113, 133].

With the aim to design high-quality joint adhesives, as above stated, the characteristics of the substrates to be bonded deserve attention as well. Indeed, the application of mechanical pretreatment on the adhesion surfaces, mainly polishing with sandpaper to increase the adhesive penetration, is generally acknowledged [231]. Besides that, Norazwani et al. [233] assessed the modification of aluminum alloy by either alkaline etching or warm water followed by silanization. The improved polyurethane adhesive wettability resulting from greater surface roughness achieved through either case was especially noticeable in the significant contact angle reduction (20–30°) and the development of shear strengths three times higher than that shown by the untreated aluminum surface.

Moreover, given the natural compatibility between isocyanate and hydroxyl bearing compounds, polyurethane adhesives have been extensively applied for bonding wood [113, 223, 232]. However, on this substrate the adhesive mobility into the substrate porous is strongly influenced by wood moisture, owing to the side reaction occurring between NCO groups with water (free or bound) instead of with substrate reacting sites. Therefore, Bastani et al. [234] thermally modified Scots wood pine to control its moisture content, while Lavalette et al. [235] revealed the greater influence of moisture content on the ultimate shear strength than the amount of one-component polyurethane adhesive in the production of green-glued plywood. Hence, the considerable influence of water contact is apparent, although it can also be extended to curing and storage steps as shown by Chattopadhyay et al. [236] or Asahara et al. [217] when analyzing the curing kinetics under moisture conditions. Because of the critical importance of correctly understanding the adhesive aging, Huacuja-Sánchez et al. [237] have studied the long-term adhesion performance of crosslinked PU adhesives by conducting different maturation conditions, namely conditioning under 90% of relative humidity at 60 °C and immersion in water at different temperatures. The plasticizing effect of water, more pronounced at greater temperatures, was proved to undermine the shear modulus in a 40% and T_g in 16.7 °C due to the substitution of free urethane and inter-urethane hydrogen bonding by hydrogen bridging with water molecules. Similarly, Raghunanan et al. [50] submitted castor oil-based PU adhesives to different controlled relative humidities, pointing out that greater environmental moisture allowed to achieve an improved mechanical resistance, owing to the stronger hydrogen bridging associated to a larger urea content.

In order to enhance the behaviour of polyurethane adhesives, among other properties adhesion strength, durability and thermomechanical resistance, the addition of modifiers has been proposed. Accordingly, Arán-Aís et al. [238] added rosin acid to the polyurethane-urea formulation as an internal tackifier to bestow a proper immediate adhesion to PVC. Malik and Kaur [105] reported the preparation of nanocomposites with the addition of titanium dioxide (TiO₂) in castor oil-based PU adhesives, entailing remarkable enhancements in adhesion strength and chemical resistance with only a 3% wt of nanoparticles, while the addition of a more moderate content of 2.5% wt of organically modified nanosilicas into hyperbranched polyurethane obtained from Mesua ferrea L. seed oil [239] almost doubled the lap shear strength results on wood (from 6.5 up to 10.8 MPa) also raising tensile strength in 10 MPa. For their part, Li et al. [171] achieved a substantial increase of 330% in the shear strength of WPU adhesives when adding a 25% w/w of ferric nitrate, used as a complexing agent in contrast with the neat WPU.

In an effort to further increase the sustainability of polyurethane adhesives, petrochemical derivatives traditionally used in the PU production have been substituted by more eco-friendly alternatives, such as polyols. For that matter, Tenorio-Alfonso et al. [240, 241] succeeding in completely replacing the hydroxyl-bearing building block with cellulose acetate and unmodified castor oil, preparing bio-sourced polyurethane adhesives whose shear, stripping and flexural resistances equated or even exceeded those found on a wide assortment of commercial benchmarks [242]. Conversely, Raghunanan et al. [50, 243] implemented more benign reaction conditions (at room temperature and in the absence of heating or inert atmosphere protection) to functionalize raw castor oil with hexamethylene diisocyanate, intended for bio-adhesive applications. Hence, Raghunanan et al. [50] highlighted the advanced adhesion strengths exhibited by the thus synthesized bio-adhesives on wood when cured at high humidity environments, given the promoted hydrogen bond content arising from the larger number urea linkages in the PU structure.

It can also be found the implementation of natural polyols, such as bio-based aliphatic poly(1,3-propylene dicarboxylate) diols [244], those resulting from liquefaction of polysaccharides [32, 34, 231] or from the modification of vegetable oils (predominantly epoxidation with further oxirane ring-opening [66, 78] or transesterification [126, 128, 129]), thus designing polyurethane adhesives with fairly competitive or even superior adhesion properties than commercial benchmarks.

Furthermore, the production of polyhydroxyurethanes through the already detailed non-isocyanate pathway has also been applied to synthesize PU adhesives, thus improving their renewability due to the utilization of sustainable materials, such as CO₂ [134, 136] or sucrose [245], while at the same time avoiding the utilization of isocyanates during their synthesis. This technique also promotes the adhesion to the substrates via secondary interactions (van der Waals forces, hydrogen bonding, etc.) due to the presence of hydroxyl groups in their chemical structure, outperforming not only the previous solvent-borne but also WPU adhesive dispersions [246, 247].

Finally, bearing in mind the likely reaction between free isocyanates with amine groups located in the proteins, the polyurethane adhesive field has also spread to the biomedical area, requiring quick curing process and biocompatible building blocks to be applied as bioadhesives in living tissues [49, 227, 248]. In response to the growing need for the rapid curing process, photo-crosslinkable technique emerges to satisfy such requirements [180], enabling the reduction of the curing times until one minute as Abdalla et al. [179] pointed out, obtaining morphologically homogeneous and biocompatible photo-crosslinkable biomedical polyurethane adhesives from polycaprolactone-diol and 2-isocyanatoethylmethacrylate.

Concluding Remarks

Given the emerging environmental concern prevailing in the industrial production of polyurethanes and the awareness of the use of non-renewable raw materials, the recent trends framed within the actions intended to reduce the detrimental effects attributable to traditional petroleum-based polyurethanes have been reviewed, with a special focus on adhesives, which are also generally associated with the release of toxic solvents during curing. Therefore, the viability of preparing polyurethanes from various natural feedstocks has been amply demonstrated. Indeed, among others, lignocellulosic biomass, primarily composed of hemicellulose, cellulose and lignin, has been proven effective in increasing the renewability of polyurethanes, due to their significant hydroxyl content, thereupon implemented as elastomers, adhesives, coatings, lubricants, etc. On the other hand, vegetable oils have emerged as promising polyol precursors. For that purpose, given their scant reactivity, most investigations have encompassed a series of chemical modifications (epoxidation, hydroformylation, trans-amidification, esterification, etc.) in order to confer hydroxyl functionality upon these plant oils, even though some natural oils already exhibit a certain hydroxyl character (lesquerella or castor oils), which allows them to be implemented without further functionalization, even achieving a complete replacement of the petrochemical polyols while maintaining their ultimate properties, when combining castor oil and lignocellulosic materials for adhesion purposes. Although the application of controlled environmental moisture has proved to optimize the adhesion properties of vegetable oil-based PUs, some other techniques, such as hybridization or addition of fillers seem to lead to even twofold lap shear strengths on wood. Moreover, pursuant to environmental protection, the preparation of polyurethane adhesives from wastes has demonstrated to yield bio-adhesives which not only exhibit competitive adhesion strengths but also improved curing response, fitting to the fast curing adhesive requirements. Likewise, the production of bio-sourced isocyanates from vegetable oils and fatty acids to be implemented in the conventional polycondensation reaction has been reported as well. Additionally, milder manufacturing conditions and more sustainable synthetic routes have been proposed to successfully substitute such traditional protocol, including the preparation of waterborne polyurethane dispersions, removing the utilization of the previous solvents as the dispersing agent, thus achieving an increase in the shear strength larger than a 300% when including just a 25% of ferric ions. Besides, the radiation-induced curing process has allowed the production of biocompatible polyurethane adhesives from acrylate monomers in the absence of volatile organic compounds and through a 1 min-length reaction procedure. On the other hand, the synthesis of non-isocyanate polyurethane adhesives by means of the polyaddition of cyclic carbonates and diamines has also allowed not only the promotion of the adhesion performance, owing to the additional hydroxyl groups, but also the fixation of environmental CO₂ to become part of those cyclic carbonates while mitigating the environmental impact of this greenhouse gas, even though NIPUs with a bio-sourced content up to 70% have been obtained when employing aminated and carbonated mimosa tannin extract.