Poly-isocyanates for Polyurethanes: An

TECHNICAL ARTICLE

Bio-Based Di-/Poly-isocyanates for Polyurethanes: An Overview Bhausaheb V. Tawade,1 Rahul D. Shingte,1 Sachin S. Kuhire,1,2 Nilakshi V. Sadavarte,1 Kavita Garg,1,2 Deepak M. Maher,1,2 Amol B. Ichake,1,2 Arvind S. More,1 and Prakash P. Wadgaonkar,*1,2 1Polymers and Advanced Materials Laboratory, Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India 2Academy of Scientific and Innovative Research, Delhi–Mathura Road, New Delhi 110025, India *Correspondence to: Prakash P. Wadgaonkar, Email: [email protected]

Abstract

O

wing to the wide range of physico-mechanical and chemical properties, polyurethanes are industrially important polymers which find multiple applications. Research and developments in the field of polyurethanes have always been linked with the sustainability issues from manufacturing to the final disposal of materials. In order to address the sustainability issues concerning the building blocks for polyurethanes, research groups from industrial and academic laboratories have been actively exploring the use of bio-based starting materials for synthesis/ manufacturing of polyurethanes. A number of research articles have highlighted the developments in phosgene-and isocyanate-free routes to polyurethanes and the utility of bio-derived diols / polyols in tuning the final properties of polyurethanes. On the other hand, relatively lesser attention has been paid so far towards design and synthesis of bio-based di-/polyisocyanates which are essential for synthesis of conventional poly u re t ha ne s of i ndu st r ia l relevance. The present ar ticle

gives an overview of the literature specifically focused on the attempts dedicated towards sustainable developments in polyurethanes via bio-based diisocyanate route.

1. Introduction Polyurethanes are of great interest due to their wide range of applications such as elastomers, fibers, foams, adhesives, coatings, sealants, etc.1 Generally, poly uret hanes are prepared by reaction of diols / polyols with di-/ poly-isocyanates. The properties of polyurethanes could be tuned by appropriate selection of the starting materials viz. diols / polyols, di-/poly-isocyanates and chain extenders.2,3 These starting materials are mostly derived from petroleum-based building blocks.1 It has been recognized that the long term dependence on petroleum as a source of monomers for the polymer synthesis is at risk4–6 The trends in the developments in the field of polyurethanes are summarised in Figure 1.

The recent research efforts in the field of polyurethane preparation are focused mainly on sustainability themes viz., : a) synthesis of key starting materials viz diols/polyols and diisocyanates starting from bio-based chemicals7–9 and b) development of non-isocyanate routes involving polymerization of bis-c yclic carbonates wit h diamines,10–14 polycondensation of bis-carbamates wit h diols and self-condensation of bishydrox yalk ylcarbamates.15,16 A large number of bio-based diols/ p ol y ol s h a v e a l r e a d y b e e n developed and are commercially available as potential substitutes for t heir pet roleum-based counterparts.17,18 The developments in the field of bio-based polyols and the corresponding polyurethanes have been reviewed.7,19 The present article is focused on developments in the area of di-/ poly-isocyanates based on starting materials derived from renewable resources.

Figure 1: Developments in the field of polyurethanes PU Today december 2017

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2. Diisocyanates for polyurethane Synthesis Di-/poly-isoc yanates are manufactured on an industrial scale by the reaction of amines or amine salts with phosgene. 20,21 D ue to t he tox ic n at u re of phosgene, several other means are being employed. The most attractive strategy employed on the laborator y scale is the use of ‘phosgene equivalents’ such as trichloromethylchloroformate (diphosgene)or bis(trichloromethyl) c a r bonate (t r iphosge ne). 2 2 , 2 3 However, it should be noted that these phosgene equivalents liberate phosgene and hence are not considered as true substitutes to phosgene. The frequently used petroleumba sed d iisoc ya nate s for t he manufacturing of polyurethanes are toluene diisocyanate (TDI), methylene diphenyl diisocyanate (M D I ) 1 , 6 - h e x a m e t h y l e n e diisoc yanate (HMDI), isophorone diisoc yanate (IPDI), 1,5 -napht ha lene d i i s o c y a n at e (N DI) , 4 4 ’diisocyanatodicyclohexylmethane ( H12M D I ) , p o l y i s o c y a n a t e , x ylylene diisoc ya nate (X DI) and hydrogenated XDI (H6XDI). Toluene diisocyanate (TDI), and methylene diphenyl diisocyanate (MDI) are the most commonly used diisocyanates with approximately 34.1% and, 61.3% consumption, respectively.24 Diisocyanates are known to be toxic compounds prepared by using ver y tox ic pho s g e ne . T he i r pr olon g e d exposure can cause health hazards as the vapours of isocyanates could enter human body by inhalation, 42

skin contact, etc25,26

2.1 Bio-based di-/polyisocyanates: There has been strong interest in using bio-based chemicals (Figure 2) such as amino acids, furan derivatives, carbohydrates, ligninbased aromatics, cashew nut shell liquid and vegetable oils as precursors for synthesis of di-/ poly-isocyanates.

Figure 2: Renewable resources for chemicals

In this context, the monomers of AA type i.e. diisocyanates and of AB type i.e. hydroxyl acyl azides have been synthesized based on bio-based chemicals. Di-/polyisocyanates and hydroxyl acyl azides reported in the patent and open literature are collected in Table 1 and Table 2, respectively. It is worth noting that, di-/polyisocyanates such as dimer acid diisoc yanate, methyl or ethyl esters of l-lysine diisocyanate and pentamethylene diisocyanate are available from commercial sources.

2.1.1 Amino acid-based diisocyanates The biomass derived amino acid, namely l-lysine has been used as a starting material to synthesize methyl or ethyl esters of l-lysine diisocyanate (MELDI or EELDI, respectively). (Table 1, entry 19, 20) l-Lysine diisocyanates have asymmetric aliphatic structure whic h leads to for mat ion of polyurethanes with the amorphous nature.27 Also, partilly bio-based diisocyanate (PBDI) (Table 1, entry 11) was synthesized form l-ly si ne a nd he x a me t hyle ne diisocyanate (HMDI) and was polymerized with bio-based diols to produce polyurethanes.28

2.1.2 Sugar-based diisocyanates Bio -based pent a met hylene diisoc yanate (PDI) (Table 1, entry 12) is one of the potential building blocks for polyurethanes with significant biocontent (71% renewable carbon) and is the first example of bio-based diisocyanate which has been commercialized. It is produced very efficiently from biomass using biotechnological and chemical processes (Scheme 1), e.g. energy efficient gas phase technolog y. The trimeric PDI which is useful as a hardener has been commercialized under trade name DESMODUR® eco N 7300 by Covestro.

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Sc heme 1: Sy nt hesis of pentamet hylene diisoc yanate starting from biomass F u r t h e r mo r e , a nu m b e r of isosorbide-based diisocyanates (Table 1 entries 6-9) have been prepared and utilized for synthesis of polyurethanes.29,30

2.1.3 Furan-based diisocyanates The first furan based diisocyanate (Table 1 entry 1) for synthesis of polyurethanes was reported by Garber in 1962. 31 Still and co-workers prepared a series of furan-based diisocyanates starting from methyl furoate.32,33 (Table 1 entry 2) Klein et. al. synthesised a reactive diisocyanate monomer (Table 1 entry 3) starting from hyd rox y me t hy l f u r f u r a l a nd d e mon s t r at e d t he c at a l y s tfree synthesis of linear-chain thermoplastic polyurethanes. 34 Further, cross-linked polyurethane foa ms we re de ve loped f rom the diisocyanate using in situ generated nit rogen, whic h is formed during conversion of the intermediate dicarboxylic acyl a zide into diisoc yanate, as a blowing agent. Gandini and coworkers synthesised a variety of furan-based diisocyanates (Table 1 entry 5) and studied their reaction kinetics towards polyurethane formation and effect of furan incorporation on the properties of polyurethanes.35–38 The study provided sufficient evidences that the furan ring is not involved in side-reactions and does not induce any specific chemical fragility in the obtained polyurethanes. PU Today december 2017

2.1.4 Lignin-based diisocyanates Lignin is one of the most abundant naturally occurring bio-polymers and is known to be key renewable resource for aromatic compounds. The c hemic a l component s obtained from lignin viz. vanillic acid, syringic acid, ferulic acid, syringol, guaiacol, and eugenol have attracted attention of polymer chemists as starting materials for preparation of difunctional monomers useful in synthesis of step growth polymers. The aromatic di-isocyanates, bis(4isocyanato-2-methoxyphenoxy) alkane and bis(4-isoc yanato2,6-dimethoxyphenoxy)alkane (Scheme 2) (Table 1 entry 21) were synthesized starting from vanillic acid and syringic acid and were polycondensed with bio-based diols viz, 1,10-decanediol and 1,12-dodecanediol to obtain poly(ether urethane)s.39,40

Sc heme 2: Sy nt hesis of diisoc yanates containing oxyalkylene linkage starting from vanillic acid / syringic acid

2.1.5 CNSL-based diisocyanates: Cashew nut shell liquid (CNSL) is

a by-product of cashew processing industry and is abundantly available in India, Brazil, Bangladesh, Kenya, Tanzania, Mozambique, SouthEast and Far-East Asia, and tropical regions of Africa. As CNSL is a non-edible oil, its utilization as an industrial raw material does not necessarily affect the food supply chain.41 CNSL has been found to be an interesting renewable starting material for synthesis of aromatic difunctional monomers due to its three reactive sites (phenolic –OH, aromatic ring and unsaturation in side chain). A range of difunctional monomers inc luding di-isoc yanates, containing pendent penatdecyl chain were synthesised. 42–44 A TDI ‘ look-alike’ diisoc yanate, n a me l y 2,4 - d i i s o c y a n ato -1pentadecylbenzene (Table 1 entry 23) was synthesized starting from CNSL ( Scheme 4).44

2.1.6 Vegetable oil based di-/poly-isocyanates: Aliphatic diisocyanates are very important building blocks for preparation of polyurethanes useful in the coatings industr y. The literature revealed that vegetable 43

TECHNICAL ARTICLE

Scheme 4: Synthesis of 2,4-diisocyanato-1-pentadecylbenzene starting from CNSL oils are attractive renewable starting materials for long chain diacids that are precursors for the preparation of diisocyanates. A range of aliphatic diisocyanates (Table 1, entries 1017) possessing long alkylene chains without or with unsatuaration in the chain were synthesized from corresponding dicarboxylic acids. The diisocyanates were synthesized on laboratory scale by conversion of aliphatic dicarboxylic acids into corresponding dicarboxylic acyl azides followed by thermal Curtius rearrangement in hydrocarbon s olve nt s . T he prop e r s a fe t y precautions are recommended to be followed due to the known hazards of handling azido compounds on larger scale and is perceived as one of the limiting factors for industrial scale manufacturing of diisocyanates via this route.

1, entry 10) DDI has 36 carbon atoms in the chain and dangling chains which are present in the structure cause steric hindrance for cross-linking and limit stress transduction.9 Table 1: Bio-based diisocyanates

Cramail and co-workers reported synthesis of aliphatic diisocyanate n a m e l y 1- i s o c y a n a t o -10 [(isocyanatomethyl)thio]decane (Table 1, entr y 16) from fatty acid by sequence of reactions such as thiol-ene, hydrazonolysis, acyl azide formation and finally Curtius rearrangement to form the diisocyanate.8

2.1.7 Bio-based AB monomers for polyurethanes The A-B type monomers namely, hydroxyl acyl azides (Table 2)

Polyisocyanate based on soyabean oil was synthesised with substitution of allylic bromide of triglycerides of plant oil with AgNCO. (Table 1, ent r y 18) Fur t her, dimer acid diisoc yanate (DDI) was synthesized from dimer of a fatty acid and is commercially available from Henkel Corporation. (Table 44


TECHNICAL ARTICLE

were synthesized based on lignin derived aromatics (Scheme 3) as well as hydroxyl acids derived from vegetable oils and were s e l f-polyconde n s e d to for m polyurethanes. The isocyanate group is generated in situ by thermal decomposition of acyl azide group which undergoes reaction with hydroxyl group resulting in

the formation of resonably high molecular weight and film-forming polyurethanes.51–53

Summary and Future Outlook: The search for renewable monomers for polyurethanes viz, polyols, chain extenders (diols, diamines)

Scheme 3: Synthesis of-hydroxyalkyleneoxy benzoyl azides and poly(ether urethane)s PU Today december 2017

and di-/poly-isocyanates is ongoing for several decades and will be long lasting. It has been demonstrated with suitable examples that biomass derived feedstocks viz., vegetable oils, cashew nut shell liquid, lingocellulose and so on hold particular promise for dramatically increasing the pool of possible inter mediates for synthesis of value added monomers and chemicals for polyurethane industry because they provide a rich array of chemical diversity. Furthermore, di-/poly-isocyanates derived from renewable resources are CO 2 neutral and could be direct “drop-in” replacements for petrochemical-based di-/polyisocyanates. However, the final utility and value of these bio-based monomers can only be assessed by carrying out the detailed enduse application validation. It is to be noted that a full-fledged academiaindustry collaboration is imperative to realise the full potential of biobased di-/poly-isocyanates in terms of their availability on industrial scale, cost ef fec t iveness and performance. New bio-based di-/ poly-isocyanates would make to the market place only if they offer real overall benefits in terms of unique and better set of properties and performance compared to existing commercial petroleumbased counterparts. Regulatory pressures are further key drivers to accelerating progress towards putting into place a road-map for bio-based di-/poly-isocyanates so as to reduce dependency of 45

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Table 2: Bio-based AB monomers containing acyl azide and hydroxyl groups

polyurethane industry on fossil fuel-based monomers. References: 1. H.-W. Engels et al., Angew. Chemie Int. Ed., 2013, 52, 9422–9441. 2. D. A. Babb, in Adv Polym Sci, 2011, pp. 315–360. 3. T. Calvo-Correas et al., ACS Sustain. Chem. Eng., 2016, 4, 5684–5692. 4. D. K. Schneiderman et al., Macromolecules, 2017, 50, 3733–3749. 5. G. W. Coates et al., Macromolecules, 2009, 42, 7987–7989. A. J. Ragauskas, Science (80-. )., 2006, 311, 484–489. 6. A. J. Ragauskas, Science, 2006, 311, 484–489 7. Z. S. Petrovic, Polym. Rev., 2008, 48, 109–155. 8. A. S. More et a., Eur. Polym. J., 2013, 49, 823–833. 9. L. Hojabri et al., Biomacromolecules, 2009, 10, 884–891. 10. J. Datta et al., Polym. Bull., 2016, 73, 1459–1496. 11. M. Firdaus et al., Green Chem., 2013, 15, 370–380. 12. C. Carré et al.,Eur. Polym. J., 2016, 84, 759–769. 13. A. Cornille et al., Eur. Polym. J., 2016, 75, 175–189. 14. H. Blattmann et al., Macromol. Rapid Commun., 2014, 35, 1238–1254. 15. L. Maisonneuve et al., Chem. Rev., 2015, 115, 12407–12439. 16. G. Rokicki et al., Polym. Adv. Technol., 2015, 26, 707–761. 46

17. D. P. Pfister et al.,ChemSusChem, 2011, 4, 703–717. 18. P. Rojek et al., J. Appl. Polym. Sci., 2012, 125, 2936–2945. 19. S. Hu et al., ChemSusChem, 2014, 7, 66–72. 20. C. Six and F. Richter, in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2003, pp. 14–16. 21. H. Ulrich, Chemistry and Technology of Isocyanates, J. Wiley & Sons, 1996. 22. H. Eckert et al., Angew. Chemie Int. Ed. English, 1987, 26, 894–895. 23. V. F. Mironov, J. Organomet. Chem., 1984, 271, 207–224. 24. O. Kreye et al., Green Chem., 2013, 15, 1431–1455. 25. H. Cotarca, L Eckert, Phosgenations - A Handbook, Wiley, 2005. 26. C. A. Krone et al.,Bull. Environ. Contam. Toxicol., 2003, 70, 328–335. 27. S. N. Goyanes and N. B. D’Accorso, Eds., Industrial Applications of Renewable Biomass Products, Springer International Publishing, Cham, 2017. 28. S. Sahoo et al., Macromol. Res., 2017, 25, 772–778. 29. F. Bachmann et al.,Macromol. Chem. Phys., 2001, 202, 3410–3419. 30. M. D. Zenner et al.,ChemSusChem, 2013, 6, 1182–1185. 31. US 3049552, 1962. 32. J. L. Cawse et al., Die Makromol. Chemie, 1984, 185, 697–707. 33. J. L. Cawse et al., Br. Polym. J., 1985, 17, 233–238. 34. C. N. D. Neumann et al., Macromol. Rapid Commun., 2011, 32, 1373–1378. 35. M. N. Belgacem et al., Eur. Polym. J.,

1993, 29, 1217–1224. 36. S. Boufi et al.,Macromolecules, 1993, 26, 6706–6717. 37. S. Boufi et al., Polymer (Guildf)., 1995, 36, 1689–1696. 38. A. Gandini, Adv. Polym. Sci., 1977, 25, 47–96. 39. S. S. Kuhire et al., Polym. Int., 2017, 66, 892–899. 40. WO2016103283 A1, 2016. 41. C. Voirin et al., Polym. Chem., 2014, 5, 3142–3162. 42. D. Chatterjee et al., in Cashew Nut Shell Liquid: A Goldfield for Functional Materials, Springer International Publishing, Cham, 2017, pp. 163–214. 43. A. S. More, Ph D Thesis, University of Pune, Pune, 2009. 44. N. V. Sadavarte, Ph D Thesis, University of Pune, Pune, 2012. 45. L. Hojabri et al., J. Polym. Sci. Part A Polym. Chem., 2010, 48, 3302–3310. 46. Y. Li et al., Prog. Org. Coatings, 2015, 86, 134–142. 47. L. Gustini et al., Polym. Chem., 2016, 7, 6586–6597. 48. T. Calvo-Correas et al., Eur. Polym. J., 2015, 70, 173–185. 49. M. Charlon et al., Eur. Polym. J., 2014, 61, 197–205. 50. G. Çaylı et al., J. Appl. Polym. Sci., 2008, 109, 2948–2955. 51. S. S. Kuhire et al., Eur. Polym. J., 2015, 71, 547–557. 52. D. V. Palaskar et al., Biomacromolecules, 2010, 11, 1202–1211. 53. A. S. More et al., Polym. Chem., 2012, 3, 1594.
