Agathe
Mouren
and
Luc
Avérous
*
BioTeam/ICPEES-ECPM, UMR CNRS 7515, Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France. E-mail: luc.averous@unistra.fr
First published on 15th December 2022
Polyurethanes (PUs) are a versatile and major polymer family, mainly produced via polyaddition between polyols and polyisocyanates. A large variety of fossil-based building blocks is commonly used to develop a wide range of macromolecular architectures with specific properties. Due to environmental concerns, legislation, rarefaction of some petrol fractions and price fluctuation, sustainable feedstocks are attracting significant attention, e.g., plastic waste and biobased resources from biomass. Consequently, various sustainable building blocks are available to develop new renewable macromolecular architectures such as aromatics, linear aliphatics and cycloaliphatics. Meanwhile, the relationship between the chemical structures of these building blocks and properties of the final PUs can be determined. For instance, aromatic building blocks are remarkable to endow materials with rigidity, hydrophobicity, fire resistance, chemical and thermal stability, whereas acyclic aliphatics endow them with oxidation and UV light resistance, flexibility and transparency. Cycloaliphatics are very interesting as they combine most of the advantages of linear aliphatic and aromatic compounds. This original and unique review presents a comprehensive overview of the synthesis of sustainable cycloaliphatic PUs using various renewable products such as biobased terpenes, carbohydrates, fatty acids and cholesterol and/or plastic waste. Herein, we summarize the chemical modification of the main sustainable cycloaliphatic feedstocks, synthesis of PUs using these building blocks and their corresponding properties and subsequently present their major applications in hot-topic fields, including building, transportation, packaging and biomedicine.
Among the different available polymers, PUs are the sixth most produced polymer family. The PU market size was valued at 71 billion USD in 2020, which is expected to reach 73 billion USD in 2028.13 The PU market is divided into various parts comprising foams, coatings, elastomers and adhesives.14 PUs are mainly characterized by the presence of urethane groups in their polymer chains. Urethane groups can be synthesized via several pathways, but the most common to date is the addition between hydroxyl group (OH) and isocyanate (NCO) groups. The corresponding polyisocyanates and polyols are mainly fossil-based and present different architectures.
IUPAC defines aliphatic compounds as acyclic, cyclic, saturated or unsaturated carbon compounds, excluding aromatic compounds.15 It is well known that aromatic and aliphatic building blocks can bring specific advantages and drawbacks to the final polymer properties. For instance, acyclic aliphatic compounds result in good hydrophobicity, high UV stability, and hydrolysis and oxidation resistance. They also present chain mobility in the polymer structure, reducing the glass transition temperature (Tg) and rigidity. This can lead to amorphous structures, which are preferred for certain applications requiring high transparency. Acyclic aliphatic building blocks result in low viscosity, easy synthesis process and avoid the use of solvents. However, linear aliphatic building blocks result in also low thermal stability, and their active functional groups are in general less reactive than aromatic ones. Aromatic building blocks give interesting polymer properties after synthesis such as stiffness, thermal resistance and barrier properties. The π–π interactions between aromatic rings increase the rigidity, Tg and mechanical behavior. However, their drawbacks include low resistance to photo-oxidation, leading to yellowing due to UV degradation and absorption. Consequently, the viscosity increases and the processability and solubility decrease.16–21 Cycloaliphatic compounds, which represent cyclic aliphatics, are a large class of chemicals, with the advantages of linear aliphatic and aromatic building blocks in terms of thermal properties, thermal stability and processability. Furthermore, the final materials properties such as flexibility, Tg, transparency and mechanical properties are intermediate between aromatic and linear aliphatic.22–27
Currently, research and production are focused on the development of new sustainable PU systems. The corresponding building blocks or/and polymers can be obtained from plastic waste or/and from biomass using chemical and biochemical (biotech) processes, which can be coupled, leading to a “chem-biotech” process.
Intensive efforts are devoted to chemical recycling, where plastics are depolymerized into intermediates and building blocks. They can be repolymerized to produce the same polymer with virgin-like properties or can be converted to obtain another polymer. If the final product is of higher value, this recycling type is called “upcycling”.28 Three main chemical processes are available for recycling plastics depending on their chemical nature, which have different advantages and drawbacks, as follows: (i) chemical reactions such as solvolysis can be applied to polymers with ether, ester and acid amide bonds. (ii) Dissolution/precipitation is applicable to mixed plastics, which may also contain additives and impurities. (iii) Thermomechanical routes such as pyrolysis are mostly applied to polyolefins.29
Due to recent biotechnological approaches, it is possible to use plastic waste as an interesting carbon source by microbial depolymerization30 or enzymatic plastic degradation31 into oligomers and monomers, which can be recovered to finally obtain polymers via a chemical or biochemical pathway. In the case of “biodegradation”, biotic degradation leads to mineralization in the presence of microorganisms, with the production of CO2 or CH4 in aerobic or anaerobic systems, respectively.32 These different short molecules can be used as alternative carbon-based substrates for microbes by fermentation to produce (i) bacterial polymers, as in the case of polyhydroxyalkanoates (PHAs), and (ii) a wide variety of valuable products such as glycolipids, aromatics, organic acids and alcohols.33
Also, biomass can be employed to obtain biobased polymers.34–37 Biobased polymers are defined by IUPAC as materials composed or derived in whole or in part of biological products obtained from biomass, with a positive C14 content.38 Biomass is based on all living biological organisms, e.g., plants, mushrooms, animals and microorganisms. According to the initial biobased resource, a wide range of polymer structures can be directly or indirectly obtained, e.g., from polysaccharides, vegetable oils, proteins, bacterial polyesters, lipids, terpenes and lignins. However, an additional synthesis step may be required.
Acyclic aliphatic feedstock such as triglycerides, e.g., from vegetable oils, animal fats and micro-algae, are advantageous to give final aliphatic architectures using oleo-chemistry. Triglycerides can be employed to easily obtain fatty acids, with numerous structural variations such as (i) chain length of up to 24 carbons, (ii) different number of double bonds (saturation, mono and poly-unsaturation), (iii) different types and numbers of other active groups (carboxylic, OH, and epoxy groups) and (iv) different stereochemistry of the double bonds. Thus, triglycerides, fatty acids and their derivatives have been intensively studied and developed for the synthesis of various aliphatic polymers in the frame of the oleochemistry.39–42
Aromatic architectures can also be obtained. The most studied sustainable aromatic resources include lignins, tannins and their derivatives. Besides their abundance and low cost, various chemical modifications are possible to develop rigid and aromatic biobased polymers due to their high OH contents.16,17,43
Based on acyclic aliphatic and aromatic compounds, a large range of sustainable cycloaliphatic resources is available to obtain different cycloaliphatic PU architectures with specific properties. This latter point is the core of the topic of this review, which is structured into different parts. The first section of this original review provides some generalities on PUs, their chemistry, and the main chemicals and the different strategies employed to increase their sustainability. Secondly, we focus on the synthesis of biobased cycloaliphatic PUs and their main properties, also presenting their main sustainable building blocks and final materials. Finally, in the last section, we introduce the main applications of these biobased cycloaliphatic PUs, e.g., foams, coatings, adhesives and membranes, including some biomedical applications.
Nowadays, the most common route to synthesize PUs is the fast and exothermic polyaddition reaction between polyols to introduce H-labile groups and polyisocyanates, with or without catalysts.47 The two main PU categories are thermoplastics and thermosets. Thermoplastic PUs (TPUs) are obtained via a reaction between compounds with a functionality of 2 (f = 2). The final polymer is soluble in different solvents and can be thermo-reprocessed. Thermoset PUs are synthesized with at least one polyfunctional reagent with f > 2. A third category is often added with covalent adaptable networks (CANs), which can be under an external stimulus, dissociative or associative, such as vitrimers without the loss of their global architectural integrity. CANs present the advantages of both systems, i.e., the recyclability of thermoplastics and the high mechanical properties of thermosets.48
The nature of the H-labile group is related to the reactivity towards isocyanates, as shown in Table 1. For instance, the different amines (from primary, secondary, aliphatic or aromatic groups) are more reactive than OH groups. Water and the primary OH group have similar reactivity. Urea, tertiary OH, aromatic OH and urethane are 2 to 300 times less reactive than secondary OH.49,50
A large variety of polyols, polyisocyanates, chain extenders and cross-linkers allows high variability in the final polymer architectures. For instance, long chain polyol brings softness and elasticity to the final PUs, whereas a high level of cross-linking results in rigidity.50
It is also possible to tailor the final properties of PUs by the incorporation of additives or modifying their processing conditions. These different variabilities result in high versatility for a wide range of applications.14
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Fig. 1 Summary of the main chemical reactions between isocyanate and H-labile groups, adapted from ref. 49. |
The reaction between isocyanate and alcohols is the main route for the synthesis of PUs. This reaction is exothermic with an enthalpy release of 100 kJ mol−1.50 Isocyanate can react with water to form an unstable carbamic acid in the first step. The decomposition of carbamic acid leads to the formation of an amine and CO2 release with an enthalpy release of 196 kJ mol−1. In the third step, the amine group reacts with isocyanate to form urea. The use of CO2 as a blowing agent and the heat produced during this reaction can be the basis for the synthesis of some PU foam, starting from room temperature.52 Isocyanates can also react with urethane to form allophanate at temperatures higher than 110 °C.53 In a similar route, disubstituted urea and isocyanate react to form biuret at temperatures higher than 120 °C. The formation of allophanate and biuret formation is reversible. Amide and CO2 are produced by the reaction between carboxylic acid and isocyanate. The further addition of isocyanate to amide leads to the formation of acylurea.54 The dimerization of isocyanates often occurs with aromatic isocyanates due to their high reactivity.55 Depending on the condition and catalysts, the reaction products can be uretidinedione and carbodiimide. Carbodiimide is produced by the thermal degradation of isocyanate at temperatures higher than 160 °C.56 In the presence of special catalysts such as potassium acetate, the trimerization of isocyanate can occur to form heterocyclic isocyanurate, finally forming polyisocyanurate (PIR), which is increasingly used to produce specific foams with higher fire resistance.50,57 In the case of the reaction between isocyanate and cyclic anhydride, cyclic imide is produced. Oxazolidones are cyclic urethanes formed by the reaction of isocyanates with epoxide compounds, under specific catalyst conditions and at temperatures between 50 °C and 60 °C.58
Polyisocyanates can be aromatic, aliphatic, cycloaliphatic or polycyclic. The main diisocyanates used in PU chemistry are as follows: (i) toluene diisocyanate (TDI), which is commercialized as a mixture of 2,4 and 2,6 isomers, (ii) 4,4′-methylenediphenyl diisocyanate (MDI) and polymeric 4,4′-MDI (pMDI), (iii) 1,6-hexamethylene diisocyanate (HDI), (iv) 4,4′-dicyclohexyl diisocyanate (HMDI) and (v) isophorone diisocyanate (IPDI). Their chemical structures are presented in Table 2. All these polyisocyanates are bifunctional except pMDI, which has an average f of 2.7. Also, they are all fossil-based and synthesized from phosgene chemistry.61,62 Given that aromatic diamines are more available and cheaper than aliphatic diamines, aromatic isocyanates such as TDI and MDI represent around 95% of the diisocyanate market for the production of PUs.63 Aromatic isocyanates are efficient given that they have higher reactivity due to the negative charge delocalization on their aromatic ring.64 The stability of aromatic isocyanates results in enhanced thermal and flame retardant properties. Aliphatic isocyanates are also useful for UV and oxidation resistance.20
In a symmetric diisocyanates, both NCO groups have the same reactivity. However, once a nucleophilic compound is added to an isocyanate group to form the first urethane group, the second unreacted NCO group reactivity is lowered. This is due to the electron-releasing effect of PUs.50,59,63 For example, the first isocyanate groups of MDI and HDI are 2.9 and 2.0 times more reactive than the second one, respectively. This reactivity gap is accentuated in the case of asymmetric diisocyanates. For example, the addition of the first OH group to 2,4-TDI is 12 times faster than the second one. Temperature and catalysts decrease the selectivity of the OH groups for isocyanates.63
Specifically, f is directly correlated with the morphology (thermoplastic and thermoset) of the final PUs and the corresponding mechanical behavior. The density of OH groups (IOH) is expressed as milligrams of KOH per gram of molecule (mg KOH g−1) and the molar mass of polyols is usually in the range of 0.5 to 10 kDa.
Polyether polyols are often produced via the ring-opening polymerization (ROP) from epoxides such as propylene oxide (PO), ethylene oxide (EO) or butylene oxide with a polyol as the initiator to obtain secondary OH. One conventional polyether polyol is polytetramethylene ether glycol (PTMEG), which has been commercialized by BASF (Germany) as PolyTHF® (PTHF) and obtained from tetrahydrofuran (THF). This soft polyol is commonly used for the synthesis of elastomeric PUs. Poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) are also broadly used. Polyether polyols have many advantages such as low cost and low viscosity, and they can endow the final PUs hydrolytic stability and flexibility. However, they present very weak oxidative and thermal stability.59,65
The production of polyester polyols is based on the polycondensation between polyhydroxyls and polyacid building blocks or from hydroxyl-acids (such as lactic acid), by ROP from cyclic lactone (lactide and caprolactone), or/and by transesterification from polyesters with a short polyol (glycolysis). There are numerous inexpensive and commercially available building blocks, but the most common for polyacids are adipic acid, succinic acid, phthalic anhydride, and polycaprolactone (PCL). The most common short polyols with f = 2 are ethylene glycol, 1,3-propylene glycol, 1,4-butanediol (BDO), and 1,6-hexane diol (HDO), such as Realkyd® produced by Arkema (France). Glycerol is largely used when f > 2 is desired to obtain the final thermosets.66 Different polyols can be combined to tailor their functionality, properties, and cost.67 The synthesis of polyester polyols is relatively easy to control to obtain primary OH end groups. Ester bonds lead to sensitivity to hydrolysis. Consequently, the final materials can be degraded under long-term exposure to moisture. PUs with polyester polyols generally have higher mechanical properties (rigidity) compared to polyether polyols due to the molecular rigidity of the ester bonds. However, these PUs need to be synthesized at high temperature due to their higher viscosity. Aromatic polyester polyols are remarkable due to their fire resistance. Aliphatic polyester polyols are used when UV and abrasion resistance are required, which allow higher deformation at low temperatures.20,65
PC polyols were initially prepared via the reaction of a diol and a phosgene. The most prevalent process is the transesterification of glycol and dialkyl or diaryl carbonates at temperatures of up to 200 °C. Another method is the ROP of cyclic carbonate with a polyol as the initiator. Poly(1,6-hexanediol) carbonate, obtained by the polycondensation of 1,6-hexanediol and diphenyl carbonate, is a PC polyol widely employed for the synthesis of PUs. Covestro commercialized it as Desmophen®.68 PUs with PC polyols are suitable for long-life applications because they are extremely hydrolysis and oxidation resistant.50,65
Acrylic polyols contain both vinyl and OH groups. They are obtained via the free radical copolymerization of acrylic or methacrylic acid esters, including methyl, ethyl, and butyl methacrylate. They can be copolymerized with functional groups such as styrene, vinyl ester and maleates. The free OH groups are usually added by ester functionalization in a second step. The properties of the final PUs depend on the nature of their monomers. For example, methyl methacrylate results in hardness and water and UV resistance compared to styrene, which is UV sensitive. Some monomers can provide flexibility, adhesion or solvent/grease resistance. However, due to their high costs, these polyols are only used for high-performance PU coating applications such as automotive refinishing.69 These polyols can be found on the market, for instance BASF (Germany) proposes a wide range of acrylic polyols named Joncryl®.70
Polyamines can also replace polyols (short or long) to react with isocyanate to synthesize polyureas. Their advantage is that aliphatic or aromatic amines react faster than OH with NCO (see Table 2) and the urea bonds present higher rigidity and thermal stability. For instance, elastane fibers are poly(urethane-urea) polymers. They are prepared from diamine chain extenders, MDI and either polyether or polyester soft segments. EDA is the most common diamine chain extender.65
The foam morphology drives the final properties and behavior. Generally, F-PUFs exhibit a low density, low compressive strength, high recovery properties and high water absorption. They are commonly used for a wide range of comfort and cushioning applications such as transportation, furniture, and bedding. They have the ability to be foamed through the slabstock process or molding.76,77 R-PUFs are mainly designed for thermal insulation and energy recovery due to their high density of closed cells. These cells contain the vaporized blowing agent with low thermal conductivity. These elements provide high compressive strength, good heat stability and good barrier properties. R-PUFs are convenient for building application because of their ability to be on-site sprayed. It is also possible to manufacture R-PUF board stocks or sandwich panels.78 Between F-PUF and R-PUF, intermediate systems can be created with intermediate behaviors.
The main PU coating technology is based on a two-component system with the reaction between polyisocyanate and polyol (usually with f > 2) at ambient temperature, with an adequate catalyst in a solvent. The coating is generally sprayed as a clear coat in automotive and aviation applications.80 To reduce the emission of volatile organic compounds (VOC), the solvent can be replaced with water for amphiphilic systems. Water-borne systems are based on dispersible polyols and aliphatic isocyanates because MDI and TDI have high reactivity with water.81 Also, a solvent-free two-component PU is available for floor coating. Low viscosity is required and fillers are added to achieve the final required properties.82 There is also an oven curing system for metal coating. A blocked isocyanate is unblocked when heating and react rapidly with a polyol to form the final coating. These systems can be solvent- or water-borne, or in powder form.83 Moisture-cured systems are interesting as anti-corrosion coatings without mixing and metering. Curing reaction occurs between an isocyanate-terminated prepolymer and water to form urea, which results in hardness and resistance to water and chemicals. However, this reaction is generally long and difficult to control because it is dependent on the room temperature and humidity.84 Urethane acrylate coatings are another one-component solvent-free system. The fast prepolymer curing is induced by the UV cross-linking of free acrylates at room temperature. This reaction is often activated by a photo initiator.85
Accordingly, PUs can also be considered as a sustainable solution to address economic and environmental concerns. For instance, in building insulation, R-PUFs are used for energy saving, which have a lightweight structure, reducing the transport and energy costs. Another example is the highly durable PU coatings, which protect and increase the shelf-life of products.92
However, the sustainability of PUs need to be increased by reducing the health issues and GHG emissions linked to their production and by considering their end-of-life. Thus, to achieve these objectives, we can find different sustainable strategies. The first global strategy is based on the valorization of plastic waste (PU and others) at the end of life, as follows: (i) conventionally by different types of recycling and (ii) by bioproduction using biotechnology strategies; in this case, the waste is used as a carbon source.93–95 The design of the polymers can be managed to ease the recycling at the end of their life such as in the case of CANs. The second popular global approach is to develop biobased PUs from biomass resources. The last approach is based on the ban of the toxic polyisocyanates to develop PUs without isocyanates, also called non-Isocyanate PUs (NIPUs).
PU waste generally comes from post-consumer products and production waste from various markets such as construction, furniture, bedding, automotive and clothing. Thus, the valorization of these products at the end of their life cycle is now essential in the context of sustainable global development. However, this valorization is limited due to the lack of efficient collection and separation processes of the different PU resources, leading to landfilling and mismanaged plastic waste, which are the main cause of plastic pollution from the land to ocean with the production of microplastics.96
Energy recovery processes such as incineration are a well-known technique, but the toxic gas and GHG emissions, such as nitrogen dioxide and CO2, and low value of the recovered products are not satisfactory, as in the case of the gasification routes for syngas production (CO and H2).97 Thus, recycling is much more desirable as a sustainable approach for dealing with PU waste in terms of the circular economy and can generate an important profit-pool according to a recent McKinsey report.98 Given that conventional mechanical recycling can be only used on some TPUs by re-melting with the loss of some thermo-mechanical properties, several additional and sustainable recycling methods have been developed. For example, chemical and biological recycling can produce monomers and oligomers from this type of plastic waste. These building blocks can be used for the (re-)polymerization of a second generation (2G) of polymers with equivalent or other applications compared to the initial polymer.29 These processes are increasingly implemented in industries, but are still not fully attractive due to the increase in cost. Thus, to overcome this economic drawback, upcycling processes can convert plastic waste into more valuable chemicals through different processes from chemistry and/or biotechnologies.99
Different techniques have been developed such as hydrolysis, aminolysis, phosphorolysis, glycolysis, pyrolysis, and hydrogenation.100,101 Hydrolysis was the first chemical recycling method developed for flexible foams. PU waste is placed at high temperature and high pressure with water to form polyols, amine intermediates and CO2. After treatment, the initial isocyanates can be regenerated from the recovered amine intermediates.101 Aminolysis involves a transesterification reaction between the urethane ester group of PU foams and ammonia or amine to produce bi or polyfunctional amines and alcohols.102 Mixing PU waste with phosphoric and phosphonic acid ester leads to the formation of a liquid product containing PU oligomers with flame retardancy, adhesive properties and UV resistance.103 Based on short polyols, glycolysis has largely been developed for the recycling of R- and F-PUFs. It is based on transesterification reaction and can take place on the urethane groups or on the ester groups in the case of PUs based on polyester polyols, at high temperature in the presence of a catalyst. In this process, it is possible to recover polyol from PUFs at 190 °C and reuse them as building blocks for the synthesis of polymers.104 Pyrolysis involves the thermal decomposition of long-chain polymers into ash, gas and light hydrocarbons such as ethylene, benzene, and aniline under high pressure, which can be used for the synthesis of chemicals. Moreover, isocyanate can be recovered after first degradation stage (100–300 °C) where the urethane bonds are broken.105 Hydrogenation is similar to the pyrolysis process, but degradation takes place under high-pressure hydrogen instead of inert gas.101
Depending on the chemical recycling process, end-of-life products and production scraps can be transformed into oligo-urethanes, polyols, amine, gas, ash, and short hydrocarbons. Among these processes, only glycolysis and gasification are conducted on a large scale. However, their major drawbacks are the initial product separation, high energy consumption (high temperature and pressure), and the need for chemicals such as organic solvents and catalysts.101
Most researchers use microbial communities to recycle PU foams and enzymes for the depolymerization of coatings and TPUs. It was observed that PU foams from polyether polyols were easily degraded by fungi due to the easy penetration of their mycelium into the pores of PUs. Alternatively, the capacity of bacteria to form biofilms was used to degrade PU foams from polyester polyols.101 Enzymatic degradation is also interesting due to the possibility to control bond cleavage and generate building blocks for re-polymerization. Some recent studies reported that urethane bonds can be bio-lysed by combining esterases and amidases. This approach showed promising results.111–113 Furthermore, different sustainable approaches have been recently developed, e.g., involving an initial biochemical step to obtain building blocks by enzymatic depolymerization, followed by a chemical step to obtain a new generation of polymers in a perpetual and sustainable “chem-biotech” cycle.111
Due to the large variety of PU complex structures, chemical and biological degradation lead to a wide variety of end products including organic acids, alcohols, aromatics, and amines such as EG, BDO, adipic acid, 4,4′-methylenedianiline (MDA) and 2,4′-toluene diamine (TDA).111 These monomers can be polymerized into PUs, polyesters, polyamides.116 MDA and TDA can be extracted and converted into MDI and TDI via the phosgene route for the synthesis of PUs.106
They can also serve as a carbon substrate for microbial communities to be metabolized into other monomers (e.g., BDO into succinic acid).117 EG can be used as a substrate by microorganisms for the production of PHAs and glyoxylic acid.118 EG has also been converted by modified bacteria into engineered extracellular building block hydroxyalkanoyloxy-alkanoate (HAA), which has been used as polyol in the synthesis of TPUs.119
These upcycling strategies for mixed plastic waste can improve the profits from cheap carbon sources and improve the carbon yield compared to other recycling processes. However, the limitations of this highly attractive and eco-friendly chem-biotech approach are the low degradation activities of microbes and enzymes. Secondly, the separation of the PU degradation product has low efficiency and is expensive. Thirdly, the toxicity of diamines such as MDA and TDA reduces the biocatalytic activity and growth of mixed cultures.116,120
In the particular case of PUs, the urethane group can show dynamic behavior depending on its nature and the urethane/free alcohol ratio via tanscarbamoylation. Moreover, it has been shown that the appropriate catalyst can enhance the associative and/or dissociative carbamate exchange mechanisms depending on its nature.130 Recent studies have proven the possibility of the reuse and recycling of PU foams by compression molding or extrusion due to this dynamic behavior.131,132
Due to environmental concerns and limited petroleum resources, there has been interest in replacing fossil raw materials with renewable alternatives, e.g., biomass. The majority of renewable carbon is embedded in plant-derived macromolecules such as cellulose, lignin, and starch or in smaller molecules such as terpenes, vegetable oils and carbohydrates or even in biogenic CO2. There are two strategies for moving towards polymers using renewable resources.5
The first pathway is based on a type of “copy and paste” strategy, developing conventional chemical bricks, but in this case from biomass, with a rather easy and fast implementation. This strategy is often based on the sugar fermentation process as in the case of the bioproduction of ethanol in high yield by alcoholic fermentation, for the synthesis of conventional polymers. As an example, ethylene is obtained from bioethanol by dehydration and can be used as a monomer for the production of biobased low-density polyethylene (LDPE) as done by Braskem (Brazil). Ethylene glycol can be obtained from biobased ethylene and acts as a diol for PET, as in the case of the “plant bottle” developed by the Coca-Cola Company. However, the major roadblocks are biomass conversion efficiency and competitiveness.133 However, this competitiveness can vary with the emergence of strong challengers, e.g., new cheap fossil resources, as in the case of the recent strong development of shale gas production (mix of C1 to C4) in North America. This fast development is often unpredictable.
The second strategy is based on the conversion (with or without chemical modifications) of renewable feedstock into novel polymers with “new” macromolecular architectures and with sometimes “new” properties and applications, compared to conventional fossil-based polymers. The diversity of biomass-derived molecules or biomacromolecules is an advantage, but it is a challenge to achieve a controlled molar mass and specific microstructure architecture to finally develop specific properties to find a market. Thus, to solve these issues, efficient catalysis, controlled polymerization process and polymer processing are required. Given that biobased building blocks or biopolymers present interesting chemical structures and are widely available, a wide range of new materials and additives with specific properties and application is now produced. For example, different biobased polylactides (PLAs) with controlled structures and properties are now produced on an industrial scale starting from biomass-derived lactic acid.5
According to the different resources, processes and approaches, numerous and an increasing quantity of biobased compounds are produced and commercialized in the last few decades, such as biobased polyols and polyisocyanates.
The growing interest in carbohydrate-containing biomass has led to the large-scale production of various biobased monomers such as EG, 1,3-propanediol (PDO), BDO, isosorbide, succinic acid, azelaic acid, sebacic acid, terephthalic acid, 1,5-pentamethylenediamine (PDA), and 1,6-hexamethylene diamine.
In the current approach to increase the renewable content in polymers, the first step is to replace the initiator polyol for the synthesis of polyethers with biobased compounds such as sorbitol or sucrose. This leads to the formation of partially renewable polyols and PUs. Fully biobased polyether polyols can be synthesized via the polycondensation of PDO from glucose (Susterra®, DuPont Tate & Lyle BioProducts, US) to produce PDO (Velvetol®, WeylChem-Germany).135,136 BASF used the biobased BDO from Genomatica (US), also known as “BioBDO”, to synthesize polytetrahydrofurane polyester polyol by polycondensation (PolyTHF®).137,138 These short diols can also be used to produce fully biobased polyester polyol via condensation with biobased dicarboxylic acids such as adipic and succinic acid such as Biosuccinium® commercialized by Roquette (France) in association with DSM (The Netherlands).139
The interest in vegetable oils, and generally on oil-chemistry producing biobased PUs has particularly increased in the last two decades. Several reviews reported the strong evolution of the synthesis of PUs from vegetable oil.49,87,140–146 The use of vegetable oil for the synthesis of polyols or polyols based on fatty acids is in full expansion because this raw material is currently abundant, cheap, and renewable with a growing production. This market was estimated to around US$7 billion in 2020 with an annual growth of 7%.147 A large range of fatty acids can be obtained via the hydrolysis of vegetable oils. They have several active and functional chemical groups such as double bonds, which can be used to develop very rich chemistry. For instance, the epoxidation of double bonds by H2O2 and the opening of oxiranes under acidic condition are the most common route for the synthesis of polyols.148 Several grades of polyols based on vegetable oils are already commercially available, such as Radia (Oléon-France) or BiOH® (Cargill-US).149,150 They can be used to improve the hydrophobicity of PUs. Dimer fatty acids are interesting because of their high molar mass. Dimerization is mainly based on the Diels Alder reaction between unsaturated fatty acids, followed by the separation of the different obtained chemicals. Mainly aliphatic and cycloaliphatic C36-dicarboxylic acids are formed. Depending on the feedstock and reaction conditions, this reaction also leads to the formation of bicyclic, aromatic dimer acids or mixture of saturated and unsaturated branched C18-carboxylic acids with pending chains to bring softness.151,152 Dimerization of fatty acid can take place to lead to the production of cyclo-polyols (Priplast™, Croda Germany) or through polycondensation with glycol to form polyester polyols (Sovermol®, BASF Germany).153,154 These polyols are also used to improve the hydrolytic stability and aging properties in PUs.155,156
Fatty acid-based isocyanates have been studied because of their attractive chemical structures based on long aliphatic chains. Soybean oil was brominated and converted by silver cyanate (AgNCO) to diisocyanate.161,162 Aliphatic diisocyanate was synthesized from oleic acid via Curtius arrangement.163,164 Castor oil was converted to aliphatic diisocyanate via a non-phosgene route.165 Diisocyanates can be produced from dimer fatty acids via the formation of dimer diamine. Dimer fatty acid can react with ammonia to be converted into dinitrile, and then be hydrogenated in dimer diamine with a catalyst.166 There are two routes to convert dimer diamine into dimer diisocyanate, as follows: (i) phosgenation167 and (ii) reacting dimer acid chloride with sodium azide.168 The most commercialized diisocyanate form dimer fatty acid is DDI from BASF for the synthesis of coatings and elastomers.140 DDI has low toxicity, low vapor pressure and high resistance to moisture and UV light.151
Polyisocyanates based on the lysine amino acid have been studied because of the non-toxic products from their degradation (amino acid), which is advantage for biomedical applications. They also have low vapor pressure, which is a key point, with these toxic compounds, for handling and processing.169–171 Polyisocyanates such as lysine methyl ester diisocyanate, EELDI and lysine triisocyanate are products from amine-terminated lysine ester phosgenation.172–174
PDI was the first commercialized biobased diisocyanate. Mitsui Chemicals (Japan) commercialized it as STABiO™.175 In the synthesis of PDI, starch is converted into PDA through biotechnological and chemical processes before being phosgenated.176–178 The trimeric PDI has been commercialized since 2015 by Covestro (Germany) as DESMODUR® Eco 7300.179 This hardener shows good resistance to aging, scratching and chemical treatment.180 Isosorbide and isomannide have been recently studied for the synthesis of diisocyanates. Also, 1,4:3,6 dianhydrohexitols are converted into the corresponding 2,5-diamino compounds before phosgenation into diisocyanate.181,182 Another route to synthesize isosorbide-based isocyanates is via Curtius rearrangement after esterification with succinic anhydride.183
Furan-based diisocyanates have been synthesized since 1962 starting from methyl furoate and hydroxymethylfurfural.184–189 Lignin is the second most abundant biopolymer and largely studied for its derived aromatic components. Lignin-derived phenolic acids such as vanillic acid and syringic acid have been used as precursors for the synthesis of diisocyanates via Curtius rearrangement.190,191
Sustainability is not only based on the origin of the feedstocks, but also about the process, use and end of life. Biobased polymers contribute to a more sustainable economy, but some drawbacks remain, including the cost and efficiency of biorefineries and the negative agricultural impacts. Thus, a combination of bioeconomy and circularity (circular bioeconomy) is needed together with emerging recycling technologies such as biological, chemical and upcyling.192
R&D on the synthesis of safer PUs without isocyanate has been strengthened over the two last decades and companies around the world are now under pressure to minimize their isocyanate consumption and develop non-toxic compounds. The most promising approach to synthesize NIPUs seems thus far to be the polyaddition of cyclic carbonates and amines, as described in numerous reviews.197–202 Given that the primary and secondary OH groups are formed by opening cyclic carbonates, NIPUs are called polyhydroxyurethanes (PHUs). One of the great advantages of this approach is the low toxicity of cyclo-carbonates, which are based on CO2 consumption during their synthesis. The cyclo-carbonate and amine components can be obtained from biobased resources, which can lead to the formation of cross-linked materials such as fully biobased PUFs.203,204 However, there are other routes to synthesize NIPUs, such as polycondensation, ring-opening polymerization and rearrangement.205 Polycondensation (or transurethanization) involves the use of a bifunctional carbamate, a diamine and a polyol. NIPUs can also be synthesized via the ring-opening polymerization of 6–7 membered cyclic carbamates, but this involves phosgene, which is harmful. Several rearrangement reactions can be employed for the synthesis of NIPUs including Curtius, Hofmann, and Lossen rearrangements, but isocyanate intermediates are formed, which is not a significant improvement over the conventional route.206
The easiest approach to produce a mixture of terpenes is extraction from turpentine. This is the volatile fraction of pine resin, with a production of 350 kt year−1.214 Rosin is the non-volatile part of resin. Terpenes are composed of a functionalized cycloaliphatic structure, which can undergo polymerization with or without prior chemical modification.215
Bacteria such as Escherichia coli possess a native metabolic pathway for isoprene, the terpene elementary unit. The bioproduction of high-value terpenes has been achieved using recombinant terpene biosynthesis modules, metabolic engineering and site-directed mutagenesis. New terpene architectures have been extended through this approach.211
Terpenes are abundant, inexpensive, biodegradable, easy to isolate and do not compete with food or feed. They have been used for various applications such as fragrances, flavors, perfumes, pharmaceuticals, drugs, dyes, varnishes, and biofuels.216 The use of terpenes as building blocks for the synthesis of polymers has become attractive, as shown by several recent reviews on this topic.217–221 The most commonly studied terpenes for polymerization are α- and β-pinene, limonene, and myrcene because they can be easily isolated from the turpentine in orange peel in reasonable amounts. Besides, their double bonds are interesting for the synthesis of polymers because they can react directly by ionic or radical polymerizations. They can also be functionalized to develop different polymers by polycondensation or ROP.220
Up to 85 kt of limonene is produced annually worldwide, which is commonly used in the fragrance, flavor and pharmaceutical industries. Limonene possesses one double bond in its ring and another one as a side group. Because limonene is an abundant and inexpensive monomer, these alkyl groups are attractive for cationic and condensation polymerization and for chemical modifications before polymerization.217
Click chemistry can be considered as a green approach using the principles of green chemistry.18 Click reactions exhibit high yields, short reaction times, high chemoselectivity and simple reaction conditions.224 Photochemical click chemistry has been carried out to synthesize limonene-based polyols (1) and (4), dithioester (2) and diamine (5) for the synthesis of PUs.
In detail, various mercaptans such as 1-thio-glycerol react via thiol-ene addition with limonene under UV radiation in the presence of a photo initiator catalyst to form polyol (1).225 R-PUFs were prepared by mixing limonene-based polyol (1) with another commercial one, and pMDI and water. High thermal stability higher than 250 °C was achieved with a Tg above 200 °C due to the rigidity of the limonene ring.226
Limonene dimercaptan (3) was synthesized as a reaction intermediate for the synthesis of polyol (4). Firstly, limonene reacted by thiol-ene addition with thioacetic acid to form dithioester (2) before being transesterified to obtain dimercaptan (3). The triazabicyclodecene (TBD) catalyst was used to reduce the reaction time compared to the original base-catalyzed reaction in a green chemistry approach.227 Limonene dimercaptan (3) has often been transformed into polyol (4) for the synthesis of R-PUFs via thiol-ene addition with alkoxylated compounds such as 2-allylphenol, eugenol and glycerol-1-allylether.226,228
PHUs have also been synthesized from dithiol (3) and carbamate monomers from hydroxamic fatty acid derivative with two double-bonded end groups by thiol-ene addition. It has been shown that the bulky structure of limonene reduced the crystallinity of NIPU, leading to amorphous polymers with a low Tg of around −13 °C.229
Thiol-ene addition has also been reported for the synthesis of diamines (5). It was used as a reaction intermediate for the synthesis dicarbamate (6) to synthesize NIPUs. Cysteamine hydrochloride reacted with limonene to from diamine (5) under UV radiation with dimethylpropylamine as the photo-initiator. Then, diamine (5) reacted with dimethyl carbonate to form dicarbamate (6) with TBD catalyst. NIPUs were synthesized from dicarbamate (6) with polyol (1) or TBD-catalyzed fatty acid-derived diols. The bulky cycloaliphatic units of limonene dicarbamate (6) and diol (1) led to the formation of short polymers with low molar masses (6150–7900 Da), crystalline domains and Tm of up to 69 °C.230
Another way to produce limonene-based PHUs is the reaction between biscyclocarbonate (8) with acyclic or cycloaliphatic polyamines.231,232 This monomer (8) was produced from limonene dioxide (7), which can be obtained by various catalytic routes.233–236 Several pathways for the production of biscyclocarbonate (8) have been studied, but the most attractive one is the catalytic carbonization of epoxide (7) with CO2. Low molar masses were obtained (from 600 to 1840 Da) for NIPUs obtained by reaction between (H) and diamines (f = 2).237–241
For cross-linked PHUs synthesized from limonene biscyclocarbonate (H) and polyamines with f > 2, the Young's modulus and Tg increased with amine functionality and flexibility. The purification of limonene biscyclocarbonate (8) by recrystallization significantly increased the thermoset properties of the NIPUs. Very rigid and brittle polymers were obtained with an increase in the biscyclocarbonate (8) content due to the rigidity of the cycloaliphatic structure of limonene.232
Some other modifications of limonene into polyol, diamine or diisocyanate can be found in the literature. However, to the best of our knowledge, the synthesis of PUs using these modified building blocks has not been reported to date. For instance, short and long diol syntheses have been described through the hydroformylation of limonene. In the second step, two routes were developed to form a limonene-based diamine. This building block can be used as a precursor for the synthesis of diisocyanates or NIPUs.242 Besides, a patent describes the modification of limonene to form diisocyanate.243 However, the production of PUs has not been reported using these different methods.
The annual global rosin production is over 1000 kt. It is composed of rosin acids (about 90%) and about 10% of by-products. Rosin acids consist mainly of isomeric abietic-type acid and its isomers (40–60%) and pimaric-type acids (9–27%), as shown in Fig. 5. Abietic acid is characterized by a rigid aliphatic tricyclic structure with one or more unsaturations.244 Its hydrophobicity allows it to be used as a water-resistant agent for paper coating, or in antifouling, adhesive and printing inks. In the last two decades, rosin chemical modifications have been studied for the synthesis of sustainable cycloaliphatic polymers.245
In the synthesis of rosin-based Pus, the MPA (10) monomer has been widely used to produce various building blocks for the production of PUs such as polyester polyols (13) and (17), or cyclocarbonates (19).
Different polyester polyols (13) have been synthesized via the esterification of MPA (10) with various diols, with different molar masses. R-PUFs were designed from pMDI, polyester polyols (13), polyether polyols, blowing agent, surfactant and catalysts. It appeared that rosin-based polyols (13) and low molar mass diols resulted in the formation of very thermally stable foams.248 Also, it seems that R-PUFs from rosin-based polyester polyols (13) were more resistant to compression and exhibited lower thermal conductivity than commercial foam due to the rigid cycloaliphatic structure of rosin.249
MPA (B) was also the starting point to design a specific rosin-based diol (17) as a chain extender for the synthesis of TPUs. Firstly, MPA (10) reacted with 4-aminobenzoic acid by imidization to form a diacid (15). Acids were chlorinated (16) with thionyl dichloride and DMF at 85 °C for 5 h. Then, EG reacted with acyl chloride (16) by esterification to form a diol (17). Prepolymers were synthesized via the reaction between IPDI and PTHF (2 kDa). Rosin-based diol (17) was used as a chain extender. Atomic force microscopy (AFM) studies showed that the HSs domains increased with an increase in the content of rosin-based diol (17) due to the rigidity of the aliphatic rings. Also, imide groups participated in inter- and intra-molecular interactions, increasing the thermodynamic incompatibility between the HSs and SSs, thus developing microphase separation.250
MPA (10) was further described as the origin of a specific rosin-based cyclic carbonate (19). MPA (10) was esterified with epichlorohydrin, and then reacted with aqueous sodium hydroxide and tetrabutyl ammonium bromide as a phase transfer catalyst to form triglycidyl ester of maleopimaric acid (18).251 This monomer (18) has been used in cycloaddition with carbon dioxide to form a rosin-based cyclic carbonate (19). PHUs were synthesized via the ROP between this monomer (19) and various aliphatic diamines or triamines. The Tg is correlated with the chemical structures of selected amines. A higher Tg (73 °C) was obtained with cycloaliphatic rigid amine (isophorone diamine) compared to the lowest (18 °C) value obtained for linear flexible aliphatic amines (triethylenetetramine).252
MPA (10) was also studied for polymer grafting to increase the thermal stability of R-PUFs due to the rigidity of rosin. MPA (10) was grafted by imidization on the amine groups of amino-functionalized polydimethylsiloxane. R-PUF was produced from pMDI, modified polysiloxane, polyether polyol, blowing agent, surfactant and catalysts. The free amino groups of modified polysiloxane reacted with isocyanates to form urea links. SSs based on polysiloxane reduced the R-PUF compression resistance, whereas rosin-grafted polysiloxane enhanced it. The improvement in mechanical properties was attributed to the rigidity of the cycloaliphatic MPA structure and imide bonds.253
APA (12) and MPA (10) were produced from levopimaric acid by Diels Alder reaction. The esterification of APA (12) with diethylene glycol gave trifunctional polyester polyol (14) for the production of waterborne PUs (WPUs). Then, rosin-based polyester polyol (14) reacted with TDI and a polyether polyol to form a prepolymer. Chain extenders were added to complete the reaction before neutralizing and dispersing the mixture in water, in a green chemistry approach. It appeared that the corresponding PU films with high rosin polyol content (14) presented improved mechanical and thermal properties, water resistance and antibacterial properties. These results are attributed to the rigid structure of rosin and cross-link density of PU network.254
Diacid dimer (20) was largely studied as a platform molecule from abietic acid for the synthesis of PUs and design of other building blocks. Diacid dimer (20) was produced via abietic acid cationic dimerization.244 Using this molecule, abietic acid-based diisocyanate (AADI) (21) was synthesized in two steps via Curtius rearrangement. Subsequently, it was used to synthesize thermoset PUs with PCL diol and N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine (TKED) as a chain extender. In this case, the HSs were composed of TKED and rigid cyclic aliphatic structure of AADI (21). The impact of the HS content in PU has been studied. It has been shown that increasing the HS content favored microphase separation. By increasing the AADI (21) and TKED content, the cross-linking density also increased. Also, the chain mobility decreased and the steric hindrance increased, leading to higher mechanical and thermal properties.255
Rosin dimer (20) was also modified into a short diol (23) for the synthesis of TPU as a rigid chain extender. Dimer (20) was chlorinated with oxalyl chloride (22) before being esterified with EG. Then, the prepolymer of MDI and polybutylene adipate (PBA) was extended with rosin-based diol (23) at different contents. Due to the high molar masses and high chain entanglements of the bulky cycloaliphatic structure of the new diol (23), the thermal stability and the mechanical properties of the final TPUs increased.256
Other polyols, (24) and (25) from abietic acid, have also been described in recent studies. Tall oil with different rosin contents ranging from 2% to 20% reacted with diethanolamine and triethanolamine secondary amines or OH groups to form polyols, (24) and (25), respectively. They have been used to extend the TPU prepolymer of MDI and polyadipate. The degree of phase separation was higher for TPU with the rosin-based chain extenders (24 and 25) compared to EG due to its rigidity.257,258
The addition of abietic mono acid during the preparation of PUs was also intensively studied. The carboxylic acid and isocyanate groups reacted to produce an amide linkage (26) for the synthesis of R-PUF with significant compression strength improvement due to the rigid cycloaliphatic structure.259 In the formulation of TPUs, the addition of rosin resin in the mixture with BDO, as the chain extender, increased the HS content and HS/SS phase separation because of the rigidity of the urethane and amide bonds, and the cycloaliphatic structure of rosin. The molar mass, crystallization time, viscosity, rheological, mechanical and adhesion properties increased with an increase in rosin content.260–262 It was shown that after a certain rosin amount (50 eq.%), the storage modulus decreased because of the rosin bulky structure, which increased the separation of the linear PU chains.263 Monomer with imide-ureas (27) were produced via a second amide reaction with isocyanate groups. These linkages were described as HSs due to their rigidity and ability to form hydrogen bonds. Similar results to that with rosin acid as the chain extender were observed, where an increasing rosin content led to a higher molar mass and viscosity.264
Recently, hydrogenated abietic acid (28) was recently used in the acrylate formulation of PUs with IPDI and 2-hydroxyethyl acrylate. The bulky and rigid cycloaliphatic structure of rosin decreased the volume shrinkage properties compared to the linear structures. Also, the imide-urea linkages (29) formed strong hydrogen bonds, increasing the adhesion to glass, PET, PVC and PC by up to 50% compared to the commercial ref. 265.
Two polyols (36, 37) have been synthesized from α-pinene-based epoxide (E) opened with diamine or polyol, respectively. Firstly, the acid-catalyzed isomerization of α-pinene led to the formation of terpinene (32).216 Then, it reacted with maleic anhydride to form terpinene anhydride (33) by Diels Alder reaction.270 ROP with epichlorohydrin led to the formation of pinene-based epoxy resin (34).271 Then, the secondary amine of diethanolamine reacted with epoxy groups of compound (34) to form a polyol (35) for the synthesis of cross-linked PUs. Flexible transparent films were obtained with Tg in the range of 5 °C to 37 °C.272 These films showed good chemical resistance and mechanical properties due to their cross-linked structure after curing. Another pinene-based polyol has been synthesized from epoxy resin (34). Epoxy groups were opened with a diol or triol to form a polyol (36) under acidic conditions.273 WPUs have been synthesized from this polyol (36) with HDI trimer. Flexible transparent films were obtained with a Tg in the range of 35 °C to 40 °C and high water-resistance and adhesion properties.274
Pinene-based diamines (31) have been designed to synthesize fully biobased NIPUs. Firstly, N,N′-diacetyl-p-menthane-1,8-diamine (30) was synthesized from turpentine with acetonitrile under acid conditions. Then, amide bonds were hydrolyzed under the catalysis of sodium hydroxide solution in EG to form p-menthane-1,8-diamine (MDA) (31).275,276 Biobased PHUs were developed by the reaction of cyclic carbonated soybean oil with MDA (31) at 150 °C for a long time (60 h), similar to the synthesis of NIPUs, and without solvent, following a green chemistry approach. Good thermal stability was obtained. Dynamic mechanical analysis and gel content measurement confirmed that the curing was uniform and the cross-linking density was higher at the used stoichiometry.277
The synthesis of pinene-based diisocyanates has been recently patented. Several routes have been studied, starting from α-pinene or β-pinene. Firstly, pinene was oxidized to pinene-derived ketone (37, 38). After a subsequent reaction step, pinene-derived diamines were formed (39, 40, 41). They were converted via a phosgene route into diisocyanates (42, 43, 44). These diisocyanates can react to form PUs.278
Neat betulin has been widely studied for pharmacological applications such as anti-cancer, anti-HIV, and cosmetic applications.281 For the extraction of biomass, several methods of sublimation with organic solvent have been studied.282 The interest in the synthesis of polymers using betulin is increasing. Indeed, its high melting point (256 °C) is due to its rigid aliphatic pentacyclic structure based on 30 carbon atoms. Besides, betulin possesses two OH groups (potential bifunctionality) and an isopropenyl group, which can be interesting for different chemical modifications.283
Betulin-based PUs have been synthesized through a two-pot route. Firstly, anionic ROP with EO occurred. After the isolation and purification of betulin with polyethylene oxide chains, step-growth polymerization with IPDI occurred in the presence of a catalyst. A one-pot synthesis route has also been developed. The catalyst switch strategy has been employed with base-catalyzed ROP and acid-catalyzed step-growth polymerization. To avoid the high crystallinity linked to long polyethylene oxide grafted chains, 1,4-benzenedimethanol has been added in different ratios with betulin. Subsequently, a molar mass in the range of 16.9 to 83.3 kDa was obtained. When the gap between two betulin units increased, hydrophobic crystalline nano-domains were formed. Thus, homogeneous hydrophobicity increased with an increase in the betulin content because of the restricted phase separation.285
TPU with cholesterol-based side-chains have been studied as a biocompatible adhesive. Firstly, a TPU was synthesized from MDI and a polyol such as PCL or PTHF. Then, a second addition of MDI was performed, allowing the grafting of cholesterol on the urethane linkage by the formation of allophanate. Consequently, this created a high-affinity surface for the attachment and adhesion of cells.287,288
As an example, the synthesis of grafted cholesterol in PU from MDI and PTHF has been reported. Cholesterol was grafted by the second addition of MDI. Cross-linked networks were obtained when MDI did not react with cholesterol, but with two urethane bonds. The pendant chains of the bulky tetracyclic cholesterol prevented the crystallization of the HSs, leading to fully amorphous materials. Moreover, the Tg increased from −63 C to −55 °C with an increase in cholesterol content due to its rigidity.289
Cholesterol is also known as a precursor of bile acids, which help to solubilize fats for vitamin metabolism. Bile acids, such as cholic acid, chenodeoxy cholic acid, deoxycholic acid and lithocholic acid are formed in the liver of mammals and birds after the metabolism of cholesterol.290 The chemical structure of cholic acid is presented in Fig. 8 (right). Cholic acids can be employed for the synthesis of fully biobased thermoset PUs. Firstly, coatings with hydrophobic properties were synthesized using EELDI as the diisocyanate and cholic acid as the polyol. Their Tg was high, ranging from 106 °C to 118 °C.290 Then, cholic acid was used as the initiator for the ROP of ε-caprolactone on its primary OH group. This polyol has been mixed with EELDI to produce biobased and biocompatible coatings. Semi-crystalline networks were obtained with Tm in the range of 45 °C to 65 °C. Furthermore, an elastic modulus in the range of 121 to 171 MPa was obtained, with an increase in elongation at break from 11% without cholic acid to 327% with the longest cholic acid-based polyol.291
Given that carbohydrates possess at least two available OH groups (f ≥ 2), they can be used as polyols or to produce polyisocyanates, diamines, biscyclocarbonates and other components for the synthesis of PUs. Among then, isohexides such as isosorbide are commercially available sugar-based diols, which have been intensively studied for the synthesis of polymers. Due to their two secondary OH groups, various functionalization and chemical modifications are possible to create renewable building blocks for the synthesis of PUs. Other carbohydrates have been used with or without chemical modifications for the synthesis of PUs such as glucose, sucrose, starch, fructose, and others.293–298
The two furan units of isosorbide endow the corresponding polymers with a rigid structure and mechanical strength, leading to a high Tg, tensile modulus and UV stability. Isosorbide is also biodegradable, soluble in water, and generally recognized as safe and non-toxic, and thus used in food packaging and medical applications.306
Isosorbide has high stability and two functional OH groups, making it an ideal and versatile platform molecule. Easy conversion reactions are possible for various applications such as green solvents, additives (plasticizers and surfactants), and renewable polymers.307
Isohexide | Diisocyanate | Other diol/diamine | Reaction conditions | T g of SSs (°C) | T m (°C) | M n (kDa) | Strength at break (MPa) | Elongation at break (%) | Modulus (MPa) | Ref. |
---|---|---|---|---|---|---|---|---|---|---|
45a | HDI | Castor oil | 160 °C, 20 min, | / | / | / | 0.8 to 12.9 | 42 to 363 | 5 to 19 | 320 |
45a | MDI | / | DBTL, 60 °C, 6 h | / | / | / | / | / | / | 370 |
45a | MDI | Radia 7282 | 80 °C, DBTL | −48 to −41 | 194 to 219 | 12 to 39 | / | / | / | 371 |
45a | DDI | PBS diol | 80 °C, 15 h | −41 to −38 | 57 to 75 | 21 to 35 | 7.0 to 13.0 | 87 to 715 | 107 to 226 | 322 |
45a | DDI | BDO | 80 °C, 24h, DBTL | −15 to −1 | 50 to 84 | 4 to 18 | / | 30 to 700 | 45 to 40 | 323 |
45a | HDI | Castor oil, aminodisulfide | 160 °C, 20 min | 20 to 29 | / | / | 25.0 | 140 to 146 | 53 to 255 | 372 |
45a | MDI | PTHF, Castor oil, BDO | DMF, 40/60/80 °C, 16 h, DBTL | −33 to −30 | 24 to 29 | 10 to 22 | 1.3 to 14.1 | 28 to 1358 | / | 319 |
45a | HDI | PCL diol | DMF, 100 °C, Cat. | / | / | / | / | / | / | 373 |
45a | IPDI, HMDI | PPG | 80 °C, 4 h, DMDEE | 54 to 68 | / | 7 to 15 | / | / | / | 309 |
45a | EELDI | PCL diol | 70 °C, 6 h, DBTL | / | / | 36 | / | / | / | 324 |
45a | HDI | PCL diol | 120 °C, 12 h | / | / | / | / | / | 374 | |
45a | HDI | PCL diol, castor oil | DMF, 80 °C, 8 h, DBTL | −43 to −33 | 30 to 36 | 59 to 80 | 9.5 to 13.9 | 596 to 1319 | / | 375 |
45a | MDI | PTHF, BDO | 75 °C, 3h then 120 °C 16 h | −64 to −56 | 168 to 206 | 19 to 57 | 10.9 to 36.4 | 264 to 668 | / | 308 |
45a | HDI | PCL diol | 120 °C, 12 h | / | / | / | / | / | / | 376 |
45a | HDI | PCL diol | 120 °C, 12 h | −48 to −35 | 32 to 56 | / | 12.8 to 37.5 | 900 to 1520 | 377 | |
45a | HDI | PTHF | 120 °C, 12 h | −47 to −38 | / | 16 to 153 | 5.9 to 19.1 | 153 to 1695 | 16 to 41 | 318 |
45a,b | MDI | PTHF, BDO | / | −60 to −50 | 147 to 199 | 16 to 27 | 24.0 to 48.0 | 462 to 570 | 8 to 39 | 378 |
45a | HDI, MDI, TDI, | Bisphenol A | NMP, 100 °C, 3 h, n-butyltin oxide hydroxide | 74 to 178 | / | 4 to 7 | / | / | / | 316 |
45a | DDI, EELDI | Dimethylol propionic acid (DMPA) | 2-Butanone, 70 °C, 4–6 h, DBTL | 18 to 58 | / | 9 to 29 | / | / | / | 317 |
45a | DDI | DMPA | 2-Butanone, 70 °C, 4–6 h, DBTL | 15 to 19 | / | 7 to 27 | / | / | / | 379 |
45a | DDI, EELDI | DMPA | 2-Butanone, 40 °C, 2–5 h, DBTL | / | 10 to 16 | / | / | / | 380 | |
45a | DDI, EELDI, HDI, IPDI | DMPA | 2-Butanone, 70 °C, 4–6 h, DBTL | 28 to 60 | / | 9 to 17 | / | / | / | 381 |
45a | HDI, MDI | BDO | DMF, RT, 24 h, DBTL | 15 to 115 | 162 to 233 | 9 to 14 | / | / | / | 310 |
45a | HDI | PC diol | 120 °C, 12 h | −37 to −33 | 63 to 70 | 56 to 126 | 13.0 to 54.0 | 955 to 1795 | 43 to 549 | 313 |
45a | HDI | PTHF, glycerin, castor oil, | DMF, 80 °C, 24 h, DBTL | 3 to 7 | / | / | / | / | / | 311 |
45a | HDI | PCL diol | 150 °C, 12 h | 31 to 42 | / | 16 to 50 | 5.4 to 9.8 | 97 to 1245 | 8.2 to 42 | 321 |
45a | HDI, IPDI | PPG | THF, 60 °C, 2 h | −63 to −24 | / | 30 to 75 | / | / | / | 312 |
45a | TDI | PPG | Foam | / | / | / | / | / | / | 382 |
45a,b | MDI, PPDI | / | Cyclohexanone, 70 °C, 24 h | 122 to 143 | / | / | / | / | / | 314 |
45a | HDI | Castor oil | / | 65 | 164 to 170 | / | 48.3 to 55.8 | 305 to 409 | 1248 to 1549 | 383 |
45b,c | Acrylate isocyanate | 1,8-Octanedithiol | / | 15 to 19 | 40 to 158 | 8 to 22 | 37.0 to 56.0 | 462 to 1658 | 3 to 350 | 326 |
45a | TDI | Chromophore | DMAc, 70 °C, 24 h, DBTL | 60 to 120 | / | / | / | / | / | 315 |
45a | HDI | PCL diol | DMF, 100 °C, Cat. | / | 78 to 183 | 224 | 0.2 to 3.9 | 49 to 306 | 1 to 28 | 384 |
45a | DDI | Oligo-PHB diol | 70 °C, 24 h | −14 to 4 | 66 to 141 | / | 0.3 to 7.5 | 50 to 843 | 0.2 to 138 | 325 |
45a | HDI | Soybean oil-based amide diol, DMPA | 78 °C, 3 h | −1 to 9 | / | / | 0.7 to 8.2 | 406 to 794 | 4 to 63 | 385 |
45a | HDI | D,L-Lactide, PEG | Toluene/DMF, 75 °C, 24 h | 38 to 43 | / | 31 to 49 | 32.3 to 52.9 | 4 to 16 | 3062 to 3588 | 386 |
45a,b,c | HDI, MDI | / | DMAc, 80 °C, 24 h, DBTL | 65 to 183 | / | / | / | / | / | 330 |
Neat isosorbide, BDO and common polyether polyols such as PTHF and PPG react with general diisocyanates such as MDI and HDI.308–310 The synthesis of PUs can be carried out in one or two steps. DBTL, dimorpholinodiethyl ether (DMDEE), and n-butyltin oxide hydroxide hydrate can be used as catalysts. Several green or common solvents such as DMF, THF, 2-butanone, cyclohexanone, NMP, and DMAc are employed.309,311–317
As an example, TPU was synthesized from HDI with different PTHF/isosorbide ratios without a catalyst, in a green chemistry approach. Isosorbide as a cycloaliphatic compound endowed the TPU with rigidity, which was translated by an increase in Tg and elastic modulus from −48 °C to −39 °C and 16 to 42 MPa, respectively, with a high elongation (around 500%) decrease.318
More recent research has been focused on using biobased polyols such as castor oil, poly(butylene succinate) diol (PBS diol) and PCL diol.319–321 Some research groups have also used biobased diisocyanates such as DDI and EELDI to synthesize fully biobased PUs.322–325 Oligo-poly(β-hydroxybutyrate) (PHB) diols have been recently developed with different molar masses and associated with isosorbide at various ratios. These polyol mixtures react with DDI as diisocyanate without solvent and without catalyst to synthesize fully biobased TPU. A wide range of thermo-mechanical properties was obtained. The crystallinity, Tg, strain at break and elastic modulus increased with an increase in isosorbide content due to its rigid cycloaliphatic structure.325
The impact of the stereochemistry of isomannide and isoidide integrated in linear PUs was studied. Isohexide-based acrylate was synthesized via the reaction between isohexide and 2-isocyanatethyl acrylate in THF with DBTL as the catalyst. This monomer reacted in a second step by thiol-Michael addition with 1,8-octanedithiol in with dimethylphenylphosphine as the catalyst. This reaction was conducted using the green chemistry concept, i.e., click chemistry at low temperature. Isohexide-based PUs showed a wide range of behaviors, for instance, isoidide-based PUs, which present tough semi-crystalline plastic behavior compared to isomannide-based PUs, which presents amorphous elastomer properties. Rheological studies proved that isoidide stereochemistry led to more linear polymer chains with less entanglement compared to isomannide.326
Diol/diamine | Diisocyanate, cyclocarbonate | Other diol/diamine | Reaction conditions | T g of SSs (°C) | T m (°C) | M n (KDa) | Ref. |
---|---|---|---|---|---|---|---|
46c | / | / | DMAc, DBTL | 118 | 145 to 194 | 8 to 12 | 328 |
47a, BDO, BDA, BDT | 48a, 48b, 48c, 49c | / | DMAc, 80 °C, 24 h, DBTL | 48 to 112 | 90 to 305 | 9 to 20 | 182 |
2,5-Bis(hydroxymethyl)furan | 50a, 50b | / | DMF, 30 °C, 24 h, DBTL | 34 to 52 | 240 to 248 | 8 to 12 | 327 |
45a, 45b | 50a, 50b | / | DMF, 120 °C, DBTL | 78 to 81 | 116 to 117 | 14 to 15 | 183 |
Commercial diamines | 51 | / | DMF, 25 °C, 12 h, cat. | −8 to 59 | / | / | 343 |
47c | Diglycerol carbonate | / | 80–100 °C, 5 h | 65.5 | / | 5 | 344 |
52a | MDI, HDI | / | DMAc, 80 °C, 24 h, DBTL | 52 to 183 | 200 to 283 | / | 329 |
45a, 45b, 45c, 52a | MDI | / | DMAc, 80 °C, 24 h, DBTL | / | / | / | 330 |
53a | pMDI | Jeffol SG | Foam | / | / | / | 228 |
54a, 54b, 54c | MDI, HDI | / | NMP/DMF/DMAc, 60 °C, 18h | 52 to 153 | 225 to 330 | / | 387 |
55a | HDI | / | 150 °C, 12 h | 7 to 23 | 33 to 40 | / | 335 |
56a | MDI | / | Foam | / | / | / | 331 |
57a | IPDI | / | Toluene, DBTL, 120 °C, 30 min | / | / | / | 332 |
57a | MDI | / | Xylene, RT, 48 h, DBTL | / | / | / | 333 |
58a | CN981 | / | 65 °C, UV curing | 58 to 62 | / | / | 342 |
59a | HDI | 3-(4-Hydroxy-3-methoxyphenyl)acrylate | Acetone, 60 °C, 5 h, DBTL | 39 to 56 | / | / | 336 |
59a | MDI | / | RT, 48 h, DBTL | / | / | 2 | 337 |
59a | MDI | PDO | RT | / | / | 30 to 47 | 338 |
60a | TDI, MDI, IPDI, HDI | Terephthaloyl chloride | Chlorobenzene, 160 °C, 4 h, cat. | 78 to 159 | / | 4 to 24 | 339 |
45a, 60a | HDI | / | Toluene, 50 °C, 2 h, Sn(Oct)2 | 56 to 59 | / | 64 to 84 | 340 |
45b, 61b | PPDI, TDI | / | DMF, 80 °C, 6 h | / | / | 4 to 17 | 341 |
62, 63 | HMDI | PEG 200 | THF, 90 °C, 5 h | −38 to −8 | / | / | 345 |
63, 64, 65 | HMDI | PPG | / | / | / | / | 346 |
63, 64 | TDI | PCL diol, BDO | / | / | / | / | 347 |
65 | MDI | PCL diol, BDO | 120 °C, 4h, DBTL | / | / | 22 to 52 | 388 |
65 | Polypropylene oxide toluene diisocyanate | / | DMSO, RT, 10 to 24 h, DBTL | −60 | / | / | 389 |
65 | HDI | PCL diol | DMSO, 80 °C, 72 h | / | / | / | 390 |
65 | HDI | Methanol, butanol, butylamine | NMP, 115 °C, 6 h | / | 99 to 129 | / | 391 |
66 | IPDI | / | THF or DMF, 60 °C, 22 h, DBTL | 111 | / | 10 to 34 | 348 |
67 | IPDI | / | THF or DMF, 60 °C, 22 h, DBTL | −8 to 149 | / | 6 to 11 | 348 |
71, 72, 73 | HDI, MDI | / | DMF, 40 or 60 °C, 48 h, DBTL | 50 to 161 | 126 to 185 | 10 to 56 | 351 |
68, 69 | HDI | / | THF, 70 °C, 6 to 60 h, DABCO | / | / | 1 to 16 | 392 |
74 | / | / | THF, 60 °C, 48 h, Zr(acac)4 | 64 | 212 | 11 | 393 |
69 | HDI, EELDI | / | DMF, 25 °C/50 °C, 24 h/72 h, DBTL | / | / | 5 to 14 | 350 |
70 | HDI, EELDI | / | DMF, 25 °C/50 °C, 24 h/72h, DBTL | / | / | 5 to 13 | 350 |
45a, 71 | HDI, MDI | PCL diol, BDO | DMF, RT, 24 h, DBTL | −60 to −3 | 40 to 54 | 20 to 56 | 394 |
75, 76 | HDI, MDI | BDO | DMF, 40 °C, 24 h, DBTL | 35 to 126 | 137 to 232 | 40 to 65 | 395 |
77 | BICMC, MDI, IDPI, HDI | 1,4-Cyclohexanediol | DMF, 60 °C, 24 h, DBTL | 119 to 250 | 199 to 265 | 21 to 61 | 396 |
78 | HDI, IPDI, BICMC, MDI | / | 1,4-Dioxane, 80 °C, 24 h, DBTL | 143 to 248 | / | 8 to 22 | 397 |
Neopentyl glycol | 39 to 162 | / | 10 to 13 | ||||
79 | pMDI | / | Foam | / | / | / | 349 |
Isohexide-based amines, isocyanates and isothiocyanates for the synthesis of PUs are reported in Fig. 9 with different ways, from 45 to 50. Using a phosgene route, diisocyanates (48a, 48b, 48c) and diisothiocyanate (49c) have been designed from diamine (47) derived from isosorbide (45a), isomannide (45b) and isoidide (45c), respectively. Poly(thio)urethanes and poly(thio)ureas were synthesized by reacting these diiso(thio)cyanates with various diols and diamines in DMAc with DBTL as the catalyst.182 Other isosorbide- and isomannide-based diisocyanates have been obtained in several synthesis steps with succinic acid and phosgene (50a, 50b). Biobased PUs have been obtained by mixing these diisocyanates (50a, 50b) with isosorbide, isomannide and bis-hydroxymethylfuran in DMF with DBTL as the catalyst to obtain biobased PUs.183,327 Also, isoidide-based monomer (46c) bearing one OH and one isocyanate group has been synthesized to produce [AB]-type homo-PUs.328
Moreover, various isohexide-based polyols have been employed in the literature for the synthesis of PUs, such as polyether polyols, polyester polyols and polyacrylates. The final building block structures are reported in Fig. 10 with different ways (from 52 to 60).
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Fig. 10 Isohexide-based polyols, polyamines and dimercaptan for the synthesis of PUs (from 52 to 60). |
Two polyether polyols were designed from isohexides for the development of various types of PUs. Firstly, an isosorbide-based ether-diol (52a) was produced in several steps, including the protection of the exo OH group by alkylation, followed by a deprotection step. Microwave irradiation was used to improve the regioselectivity and reduce the reaction time during the alkylation step. Various aromatic and linear diols were used for this alkylation step such as bisphenol A, thiodiphenol, BDO, HDO, and PEG. These isosorbide-based ether-diols (52a) were successfully polymerized with MDI or HDI.329,330 Secondly, the oxypropylation of isosorbide led to another polyether polyol (56a) with various OH indexes. R-PUFs with improved thermal and dimensional stabilities were obtained compared to PPG as the polyol.331
Polyetheramides were also synthesized for the preparation of PUs for efficient applications. Polyetheramide polyols (57a) were designed from fatty acid and isosorbide. Firstly, fatty acid was converted to fatty amide by amidation with diethanolamine. It reacted in a second step with isosorbide to form polyetheramide polyols (57a). These products were used for the efficient synthesis of coatings.332,333 Dietheramides (54a, 54b, 54c) were developed from the reaction between three isohexides isomers with 4-fluoronitrobenzene, which were used for the synthesis of polyurea with high thermal resistance.334
Also, four biobased isohexide-based polyester polyols were produced via esterification reaction or ROP. Firstly, the reaction of 4-fluoronitrobenzene with the endo group of isosorbide led to the formation of amino-alcohol, which was used as an initiator for the ROP of ε-caprolactone on its primary amine group. These polyester polyols (55a) were used for the synthesis of PUs with HDI.335 Secondly, a condensation reaction was performed for polyester polyols (59a) between isosorbide and various dicarboxylic acids such as α-ketoglutaric acid, dimer fatty acid and azelaic acid. TPUs and coatings were prepared using these polyester polyols and MDI or HDI.336–338 Then, isosorbide was also used by several groups as an initiator for the ROP of D,L-lactide to obtain polyester polyols (60a). Various diisocyanates have been employed for the synthesis of TPUs with increased molar masses and mechanical properties.339,340 Recently, an isomannide-based polyol (61) with oxide ending groups was synthesized via the pathway described in Fig. 11. This polyol was mixed with p-phenylene diisocyanate (PPDI) or TDI to form biobased cycloPUs. It has been showed that the diisocyanate source has a strong impact on the morphology of the final material.341
Recently, isosorbide di(meth)acrylates (58a) were designed from (meth)acryloyl chloride and isosorbide. They were mixed with acrylic PUs to prepare UV curable coatings with a high storage modulus and good impact properties.342
In a sustainable approach, biobased polythioether was prepared by click chemistry from isosorbide dithiol (53a) for the preparation of PUF. This new polyol structure was developed via photochemical or thermal thiol-ene reaction with alkoxylated aromatic phenols containing double bonds such as ortho allyl phenol and eugenol.228 Also, to avoid the handling of the harmful phosgene and isocyanate, isosorbide-based PHUs have been developed via the addition of isosorbide-based biscyclocarbonate (51) with various available linear or cycloaliphatic diamines.343 The synthesis route is reported in Fig. 12. Another NIPU was synthesized from isoidide diamine (47c) with diglycerol dicarbonate.344
Due to their different OH groups, maltose (62), sucrose (63), glucose (64) and starch (65) have been intensively used as renewable cross-linkers in PUs.345–348
Modified carbohydrates have also been designed to be used as biobased cross-linkers for the synthesis of thermoset PUs. Firstly, the esterification of the primary OH group of sucrose (63) and glucose (64) has been performed with fatty acid to form new biobased cross-linkers (66 and 67), respectively. Also, a highly functional polyether polyol (79) has been designed via the oxypropylation of sucrose (63). R-PUFs were prepared using these polyols and polymeric MDI. High physical and mechanical properties were obtained.349
New OH-bearing PUs with high hydrophilicity and degradability have been designed from carbohydrates due to their hydrolysable acetals. The OH group protection of various carbohydrates by acetalization can occur, leading to the creation of cycloaliphatic structures, as presented in Fig. 13 (from 69 to 78). The starting carbohydrates were glucose, theritol, myo-inositol and d-galacto-1,4-lactone, a galactose derivative. It appeared that the thermal stability was higher for PUs with free OH groups than for that with protective acetals.350 It has also been observed that PUs from D-gluco, galacto, and D-manno building blocks (71, 72, 73) with protected secondary OH groups had similar properties to that of PU from isosorbide, respectively.351
The synthesis of PUs via oleochemistry has become a hot topic in the last two decades to obtain thermosets or TPUs due to the abundance, very low cost, non-toxicity and attractive chemical structures of vegetable oils.355–358 Their chemistry is rich. For instance, their double bonds can be functionalized into OH groups and form polyols using different approaches. Ozonolysis, epoxidation, hydroformylation, transesterification, amidation and thiol-ene coupling are the most developed methods for the functionalization of double bonds for the synthesis of polyols using vegetable oils.359
Dimers of diisocyanate, polyol, diamine, and dicyclocarbonate with cyclohexene ring can be produced via the dimerization of fatty acids, e.g., Diels Alder cycloaddition [4+2] of linoleic and linolenic acids at high temperature.360 The pending aliphatic chains of these dimers can bring mobility to the final polymers. It is also possible to eliminate the residual double bonds by hydrogenation to increase the resistance of polymers to UV light, temperature, and oxidation, consequently increasing their shelf life.323
The condensation of a dimer fatty acid with excess linear diols has been conducted to prepare new biobased polyester polyols from vegetable oils. This building block was used in addition to MDI and BDO to produce TPU. Molar masses in the range of 15 to 38 kDa were obtained depending on the HS content. Organized segregation in microdomains was observed, with the highest interaction between HSs and SSs at the highest HS content.361,362 Polyester polyol was synthesized from dimer fatty acid and 2,2-dimethylpropane-1,3-diol by esterification. WPUs have been produced from this polyol and MDI.363
The synthesis of NIPU from vegetable oil derivatives has also been studied to avoid the handling of the harmful isocyanate. Cyclocarbonates and diamines from dimer fatty acids have been designed and reported, which will be presented in the following section.
Dimer-based diamines can be obtained from dimer fatty acids via two different routes, i.e., direct amination and amidification with diamine excess. However, the second route requires the removal of a huge excess of diamine. Croda (Germany) developed several Priamine©, dimer diamines synthesized from dimer fatty acids, called Pripol©. This hydrophobic diamine exhibited low viscosity, which is interesting for solvent-free formulation and synthesis.364 The amine ratio and functionality strongly impact the properties of the final polymer. Indeed, the quantity of trimer fatty acid side products directly impacted the cross-linking density and resulted in specific structuration.365
Thermoplastics and thermoset PHUs were synthesized using sebacic biscyclocarbonate and dimer-based diamine with various functionalities and contents. Amorphous polymers were obtained with a Tg in the range of −23 °C to −14 °C.366 Sustainable PHUs were designed via green chemistry approaches, without solvent and catalyst. In this case, a fatty acid dimer was chlorinated before being esterified with glycerol carbonate to form a biobased biscyclocarbonate, with trimethylamine as the catalyst. The properties and molar masses of the final NIPUs were consistently lower than that of conventional PUs. Aminolysis reaction has lower reactivity than urethanisation between isocyanate and primary alcohol, leading to longer reaction time with lower molar masses.365
The esterification strategy was used with glycerol carbonate without the halogenation step to produce dimer-based biscyclocarbonate. PHUs and water-borne NIPUs were prepared from this building block and dimer diamines through bulk polymerization and mini-emulsion process, respectively. However, the hydrolysis of cyclocarbonate occurred during the mini-emulsion process, leading to the formation of low molar mass polymers. All the NIPUs exhibited a low Tg due to the long linear fatty chains presented in both components.367
Dimer-based diamines can be converted into diisocyanate via the phosgene or hydrazine routes.165 The Cognis-BASF (Germany) Company developed heptyl-3,4-bis(9-isocyanatononyl)-1-pentylcyclohexane (abbreviated as DDI) under the trade name DDI1410® from linoleic acid. Its purity is higher than 90% with a non-negligible amount of mono-functional isocyanates.317 It is also possible to obtain and isolate trimers as co-products from Diels Alder cycloaddition.368 The synthesis of several PUs has been realized using DDI as a flexible diisocyanate to create polymers with a high biobased content.
DDI was used as a soft building block for the synthesis of biobased TPU. Isosorbide and BDO acted as HSs. All the polymers showed HS/SS segregation by SAXS with separate isosorbide and BDO domains.323 The same research group studied the preparation of biobased TPUs from DDI, HDI, oligo-PHB diol and various chain extenders. The final biobased contents were in the range of 78% to 87%. DDI has been used as long and flexible hydrophobic parts, whereas HDI was used to increase the crystallinity. It is interesting to note that DDI was considered as 86% participating in the SSs due to its flexible C36 chain. The remaining 14% was considered as HSs, mainly based on the urethane group parts. The mechanical results showed higher Young's moduli and lower elongation at break for the TPU with longer prepolymers and highly reactive chain extenders.369
Flame retardancy has become a hot topic in the last decade, and particularly for PU foams.398 Many strategies have been employed, including the use of additives and halogen-, nitrogen- or phosphorous-based compounds for the synthesis of polymers, especially R-PUFs. For instance, rosin has been intensively studied as a flame retardant agent in R-PUFs. It was shown that rosin-based foam was less flammable and required less than half the amount of flame retardant than that with conventional polyol.399
Various other rosin-based polyols have been developed for the synthesis of R-PUFs with improved thermal stability, flame retardation and compression resistance compared to conventional flame-retardants for R-PUFs.400–402 Grafting or combining MPA or FPA with phosphorus, polysiloxanes or other halogen-free compounds further improved the flame retardancy of R-PUFs.253,403–405
Rosin has been intensively developed in WPUs for biomedical application due to its antibacterial activity.406–408 WPUs from rosin-based polyester polyol (14) were developed with improved mechanical, thermal, water resistance and antibacterial properties. The activity of the antibacterial films increased with an increase in content polyol (14) because it degraded the outer membrane of the bacteria, eventually leading to cell death. Furthermore, water absorption was reduced from 80% to 15% due to the increased cross-linked density and presence of unsaturated double bonds, providing a greater barrier to water migration.254
Due to the hydrophobicity of rosin, various WPU coatings with low water absorption and high mechanical properties have been designed using rosin. For example, the addition of 30 wt% of maleopimaric acid polyester polyol (13) to the WPU formulation, improved its thermal degradation from 170 °C to 237 °C and tensile strength from 7 to 23 MPa and decreased the water absorption from 79% to 15%.409 Two different smooth and flexible WPU transparent films were also designed using rosin-based polyols (13 or 15). It appeared that the impact strength, hardness, water-resistance and thermal resistance of these films increased with an increase in NCO/OH ratio.274,410 Two-component WPU (2K-WPU) films with low water absorption have been prepared from rosin-based polyol with f = 3. The impact of rosin-based polyester polyol content on the properties of the WPUs were investigated. The water absorption decreased with an increase in rosin polyol content due to the hydrophobicity of the rosin cycloaliphatic structure. Consequently, the tensile strength improved from 7 to 16 MPa and the soluble fraction decreased due to the increase in cross-linking density caused by the trifunctionality of the rosin-based polyol. Also, gloss and hardness were improved with rosin polyol content.411,412
Recent works have investigated isosorbide modification for the preparation of PU coatings. It has been shown that the isosorbide cycloaliphatic structure increased the thermal stability, Tg and antimicrobial properties.333,337,342 Then, isosorbide di(meth)acrylate has been designed as a photocurable reactive diluent for UV-curable coatings. It was mixed with PU acrylic to test its diluting ability. The results were similar to that obtained with a commercial diluent. Moreover, the Tg, hardness, storage modulus, and impact properties increased. Nevertheless, the bulky structure of isosorbide allowed segmented fluidity in the network, leading to the formation of flexible high-performance photosensitive resins.342
Biobased PU coatings were developed from IPDI, PLA-based diol, BDO and antifouling additives such as rosin and non-toxic butenolide. The release of butenolide was responsible for the antifouling properties. The addition of rosin delayed the release of butenolide by increasing the polymer self-polishing. Strong antifouling properties were maintained for at least two months.419
Multiblock-like amphiphilic PUs have been synthesized using betulin and ethylene oxide. PUs with different betulin contents have been spin-coated for protein-resistance tests. The results showed that an increase in betulin content decreased the phase separation and increased the homogeneity of hydrophobicity. It improved the broad-spectrum protein resistance. Indeed, when the betulin content was too low, hydrophobic crystalline domains were formed due to phase separation, providing enough sites for protein adsorption. This type of hydrophobic and protein-resistant polymer can be interesting for antifouling applications.285
TPU sealants containing rosin as a chain extender were developed as sealants for defects in disc regeneration surgery. In the case of a mixture with BDO as the chain extender, when the rosin content increased, the urethane and urethane-amide HS content increased as well as HS/SS segregation. The storage modulus decreased with an increase in the rosin content due to the bulky chemical structure of rosin compared to the linear BDO.263,421
Biomedical TPU tissue adhesives were designed from oligo-PHB diol, DDI, HDI and various chain extenders. A mixture of flexible DDI and hard HDI resulted in mid-range properties. TPU adhesive systems were applied on different substrates for lap-shear adhesion tests. Bovine muscle, liver tissues and porcine skin tissues have been tested for biomedical application. Higher reactive chain extenders showed improved adhesion on muscle tissues. Highest adhesions were obtained on skin substrate.369
Hydrogenated rosin has been studied in PUA systems, and their adhesion to different substrates tested, such as glass, PET, PC and PVC. The photopolymerization rate was impacted by a larger stereoelectronic effect and higher Tg due to the bulky cycloaliphatic chemical structure of rosin. The adhesion on these different substrates increased with an increase in the rosin-based content. This was due to the small volume shrinkage during UV-curing, leading to less internal stress. Also, imide-urea bonds were created after the second addition of isocyanate. These bonds formed strong hydrogen bonds with the different substrates, especially with polar ones such as glass, increasing the adhesion.265
Rosin-based polyols (24, 25) have been compared to EG as chain extenders in the TPU formulation with MDI and polyadipate. The degree of phase separation was higher for TPU with rosin-based chain extenders (24, 25) compared to EG. The TPU with polyols (24, 25) were stiffer due to the rigidity of rosin, leading to a higher storage modulus and thermal stability and lower relaxation potential.257,258
In the case of isosorbide-based polyester polyols, TPUs (60a) were synthesized with different diisocyanates such as TDI, MDI, IPDI, and HDI. Isosorbide was used to obtain a high Tg in the range of 78 °C to 159 °C. SnCl2 catalyst combined with aromatic solvent such as chloro-benzene or xylene was the best effective combination in this case. Indeed, it is known that metal chloride catalysts are the most efficient for condensation reactions. With a too high excess of diisocyanate compared to the conventional stoichiometry, the Tg increased and allophanate formation was favored, leading to the formation of 3D cross-linked structures. Due to the high content of H-bonds, the thermal stability decreased with excess isocyanate and the adhesion to polar surfaces and solubility in polar nucleophilic solvent increased.339
Electrospinning has also been used to prepare highly elastic scaffolds composed of HDI, isosorbide and polycarbonate diol. High thermal stability and mechanical properties were obtained due to the rigid cycloaliphatic structure of isosorbide with high elongation at break between 365% and 953%. Moreover, the hydrophilicity of isosorbide increased the cell adhesion and proliferation. However, these TPUs were too stiff for soft tissue application.374,376 In one case, it has been shown that cycloaliphatic building blocks in TPU scaffolds slowed down their hydrolytic degradation. Indeed, DDI-based urethane groups are less hydrolysable than esters groups due to the hydrophobic nature of the dimer.369
SMPUs with improved properties have been synthesized from highly functional rosin-based polyester polyols with different amounts of HDI and DBTL as the catalyst. The mechanical properties of the PU network improved with an increase in isocyanate content by increasing the crosslinking density and HS content. With the HDI content increased from 30% to 80%, the tensile strength and toughness increased significantly and the elongation at break decreased from 119% to 61%. These PU networks showed improved mechanical properties compared to previously reported ones due to their aliphatic rings and the combined effect of physical and chemical crosslinking. Concerning self-healing properties, network rearrangement took place by transcarbamoylation at high temperature. After 4 h at 160 °C, PU with 70% HDI exhibited good self-healing and welding capabilities. For the thermal-responsive shape memory tests, the initial state was reached within 1 min. Pictures of the shape-recovery performance of PU containing 70 mol% of HDI and 30 mol% of rosin-based polyol are presented in Fig. 15. These material properties were reached because of the topological rearrangement due to transcarbamoylation exchange reactions, catalyzed by DBTL, chemical and physical crosslinking networks, and a suitable content of HSs and SSs.425
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Fig. 15 Shape memory performance of PUs with 30 mol% of rosin-based polyol and 70 mol% of HDI after different treatments. Reproduced from ref. 417. |
Other renewable dynamic isosorbide-based poly(urethane-urea) networks with high mechanical properties have been designed using isosorbide, castor oil, and 4-aminophenyl disulfide. Isosorbide was incorporated to reduce the crosslinking density with the same mechanical behavior due to its rigidity brought by its cycloaliphatic structure. Good thermal resistance and high mechanical properties were obtained with a Young's modulus of 255 MPa, tensile strength of 25 MPa and elongation at break of 140%. With associative disulfide exchange reaction, these materials were could be reprocessed at high temperature. Fully recovered mechanical properties were obtained after three thermal remolding cycles at 180 °C.372
Dynamic biobased PHUs have been synthesized from pinene-derived diamine (31) and cyclic-carbonated soybean oil. Pinene-derived amino groups catalyzed the synthesis reaction and flexible soybean oil chains and pinene-derived carbamate bonds catalyzed transesterification reactions, which are responsible for dynamic bond exchange. No solvent or catalyst was added in the green chemistry approach. SMPUs with a stoichiometric ratio between pinene-based diamines (31) and cyclocarbonates showed the best thermo-mechanical properties due to the high cross-linking density. The tensile strength increased to 2.2 MPa and the elongation at break decreased to 106%. SMPUs with a high content of pinene-derived diamines (31) had longer relaxation time and lower relaxation rate due to the rigidity of the cycloaliphatic structure, which limited the chain mobility. Transesterification and transcarbonation reactions led to materials with self-healing, reprocessability, shape-memory properties and chemical recyclability. Chemical recycling in ethanol was possible due to the exchange reaction with OH groups from ethanol. Triple-shape memory behavior was possible because of the additional reversible phase transition. At 140 °C, a topology-freezing transition (or dynamic bond exchange) occurred and the material could be reshaped to its initial form. After several reprocessing cycles, the mechanical and thermal properties were almost completely recovered at more than 85% of the original value, as shown in Fig. 16.277
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Fig. 16 (a) Chemical recycling of cured CSBO-MDA1.0 PHUs. CSBO corresponds to cyclic-carbonated soybean oil. Ratio between CSBO and MDA is 1.0. (b) TGA curves and (c) stress–strain curves of original and recycled CSBO–MDA samples. Reproduced from ref. 272. |
PUs containing grafted and free cholesterol have been synthesized from MDI and PTHF. Cholesterol was grafted by the second addition of MDI, with the formation of allophanate. Cross-linked networks were obtained when MDI did not react with cholesterol, forming two urethane bonds. The tensile stress increased with an increase in cholesterol and cross-linking density from 18 to 47 MPa. The elongation at break decreased from 2013% to 1609% when the cholesterol content of increased because cholesterol hindered the stretching of the PU. Shape-recovery tests were conducted at 0 °C and 45 °C. When the amount of grafted cholesterol increased, the shape recovery at 45 °C and 0 °C increased, due to physical and chemical cross-linking. However, when free cholesterol increased, the shape recovery at 45 °C and 0 °C decreased, with premature breakage of PUs with a higher cholesterol content. The absence of chemical and physical cross-linking of PUs with free cholesterol led to a decrease in shape recovery.289
Rosin-based diisocyanate (21) has been used as a building block for the synthesis of thermo-responsive biobased SMPU. It has been shown that the microphase segregation increased with an increase in rosin-based diisocyanate (21) content. Also, increasing the cross-linking density resulted in improved thermal and mechanical properties and shape-recovery performance. The elastic modulus and Tα (relaxation temperature associated with Tg) increased from 2.2 to 3.3 GPa and from 61 °C to 78 °C, respectively. The shape fixity ratio and shape recovery ratio increased from 98.3% to 99.4% and from 96.7% to 98.8%, respectively. Also, these SMPUs showed higher hydrolytic degradability in buffer solution without enzymatic catalyst, reaching up to 71% weight loss within 8 weeks due to their numerous hydrolysable groups.255
Three different isosorbide-based SMPUs were developed for biomedical application. Firstly, SMPUs with enhanced mechanical properties were designed from two diisocyanates composed of HDI and isosorbide units. Poly(DL-lactic acid) diol was selected as the soft chain polyol and isosorbide as the chain extender. Excellent mechanical properties were obtained due to the increase in HS content. The strength at break, elongation at break and Young's modulus ranged from 32 to 53 MPa, 4% to 16% and 3062 to 3588 MPa, respectively. Moreover, good shape-memory characteristics were achieved with a shape fixity ratio of up to 99.8% and recovery ratio of up to 90.2%. In vitro degradation tests were successful with almost total degradation within 120 days. Thus, high-value bone repair application can be envisaged.386 Secondly, SMPUs based on isosorbide, castor oil, PCL diol and HDI have been designed for biomedical application. Isosorbide and castor oil were used to form net points, and PCL diol acted as the switching segment. High shape-memory properties were obtained with 95% shape recovery ratio and 90% shape fixity ratio. Moreover, shape recovery tests in 37 °C water bath were successful with shape recovery within 20 s, as shown in Fig. 17. Due to their high cell viability and proliferation, these materials can be envisaged for stent with self-expansion. The proof of concept test showed the self-expansion of the stent within 18 s.375 Thirdly, thermally responsive SMPUs from isosorbide, MDI and different amounts of PCL diol have been designed. The PU containing 30% of PCL exhibited good shape-memory properties. 95% shape-fixing ratio was obtained due to the PCL crystalline domains. The 71% shape-recovery ratio was due to the amorphous PU domains due to the bulky structure of cycloaliphatic isosorbide. This material could be knotted by itself in a water bath at 40 °C, illustrating its potential for smart suture application. Bioresorbability characteristics, cell adhesion and proliferation were proven, confirming its high potential for application in the biomedical field.377
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Fig. 17 Self-expansion of stent made with PUs from isosorbide, PCL diol, castor oil and HDI in 37 °C water bath. Reproduced with authorization from ref. 347. |
The synthesis of bioresorbable PU microspheres has been investigated with rosin as the raw material. The degradability and compatibility of rosin in the human body make it an excellent candidate for adsorption and drug release applications. Rosin-based polyester diol was used for the synthesis of WPUs with pendant CC. The addition of styrene led to the formation of microspheres. The effects of temperature and pH on their absorption properties were investigated, and it was found that they can be used for pH-sensitive drug-release application.427
Hollow-porous rosin-based PU microspheres have been developed for drug release applications. Also, a rosin-based polyester polyol was used for the synthesis of PUs. The effect of the chain extender type and amount and impact of solvent were investigated. The morphology, particle size, particle distribution and buffer volume of the microspheres were impacted by these parameters.428
PU networks from betulin, using triphenylmethane triisocyanate as a cross-linker have also been prepared. Three rigid aliphatic cycles of the betulin introduced frustrated packing. This high free volume allowed the synthesis of microporous PU to be envisaged. Specifically, 10 μm microglobules were formed by precipitation during synthesis. This morphology is suitable for gas adsorption membrane applications. The pore size was between 0.5 and 1 nm, which is interesting for the gas separation kinetics. These nanosizes can be reduced by hydrogen bonds due to the urethane linkages. Promising CO2/N2 gas selectivity was observed by adsorption studies.429
Isosorbide-based microcapsules have been prepared via interfacial polycondensation. Naphthol ether fragrance was encapsulated in a shell composed of isosorbide and MDI. Impregnation was chosen to apply these microcapsules on textile fibers. Isosorbide was chosen to replace the conventional and toxic bisphenol A. Due to the hydrophobicity of MDI and the hydrophilic character of isosorbide, microcapsules were successfully synthesized. They presented a unimodal volume distribution, with the average particle size of 27 μm. Impregnation of the textile fibers was achieved with the absence of aggregates and good resistance to washing.370
These building blocks possess different types of reactive and modifiable groups such as carbon–carbon double bonds, OH groups and carboxylic acids. Many strategies have been developed to transform these chemical structures into functional building blocks for the preparation of sustainable PUs. In the case of carbon–carbon double bonds, a large and rich chemistry can be developed such as click chemistry with Diels Alder or thiol-ene addition, considering the principle of green chemistry. In the case of OH groups and acids, various methods of esterification and etherification have been employed to synthesize new cycloaliphatic polyesters or polyether polyols. They partially or totally replaced conventional linear polyols, chain extenders and cross-linkers. The rigidity brought by the cycloaliphatic structure results in an increase in Tg and higher thermal stability of novel PUs compared to linear aliphatic polyols. In the frame of the synthesis of conventional PUs and contrary to polyols, only few renewable and sustainable cycloaliphatic diisocyanates have been developed, which are rarely described in the literature. Their synthesis require many steps, including the handling of the harmful phosgene. Some non-phosgene routes have also been described with low yield. However, in a sustainable development strategy, renewable cycloaliphatic polyamines and polycyclocarbonates were also developed for the synthesis of NIPUs. Mostly low molar mass PUs have been obtained because it is difficult to get the right stoichiometric conditions. The second reason is that aminolysis has lower reactivity than urethanisation.
These sustainable and renewable cycloaliphatic building blocks bring interesting properties to the final Pus, as follows:
• Stiffness
• High thermal stability
• Multiphasic structure associated with HS/SS micro segregation
• Low surface energy
The stiffness and thermal stability caused by the cycloaliphatic structure enable the production of PUs with enhanced mechanical and thermal properties for various applications such as R-PUFs, where these properties are particularly required and associated with flame-retardant properties, e.g., building insulation and high-performance coatings with resistance to hydrolysis and oxidation.
Novel multiphasic structures can be created due to the H-bonding ability of the functional groups of these building blocks, enabling the creation of TPUs with hydrophobic crystalline HS domains, with increased HS/SS phase segregation. This physical cross-linking improved the mechanical properties of TPUs and H-bonding ability also intensively used in the field of adhesives.
These HS crystalline domains combined with bulky apolar structure provide PUs with low surface energy. They present hydrophobicity and low adhesion of bacterial cells. Materials with high cross-linking density have enhanced transparency, gloss, and hardness.
Cycloaliphatics combine the main advantages of aromatic and linear aliphatic compounds, but some drawbacks still remain. Due to their specific structure, the high steric hindrance of the functional groups reduces their reactivities and often requires specific reaction conditions with chemical modification to obtain higher reactive groups before polymerization. Moreover, moderate barrier properties were obtained in the case of inhomogeneous structures with segregation. These morphologies result in different diffusion pathways, decreasing the permeability properties. Also, due to their bulky and rigid structure, the viscosity may increase, causing difficulties in processing, for instance, during the fabrication of the foams or coatings. Similarly, a higher Tg can make PUs brittle.
Given that the environmental impact is increasingly considered, PUs from cycloaliphatics with improved environmental impact have been designed such as renewable SMPUs, WPU coatings, antifouling coatings and UV-curing adhesives and coatings. The trend and perspectives in this field are also based on green chemistry to produce materials with high sustainable carbon content, extended shelf life and controlled end of life. Biomass is a well-known renewable feedstock with increasing control, but other sustainable carbon sources have been investigated, such as plastic waste. Several processes for chemical and biological recycling have been developed, but they are still less commercially attractive compared to low-cost virgin compounds. Thus, to overcome this key point, research on upcycling is increasing to convert plastic waste into value-added chemicals through biotechnology, which is associated with chemistry or not.
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