Vegetable oils: a source of polyols for polyurethane materials

This manuscript is dedicated to the literature on vegetable oil-based polyurethanes via the isocyanate/alcohol route. A lot of efforts have been made to replace petroleum-based resources. Among renewable resources, vegetable oils present various advantages going from their availability to the large range of possible chemical modifications they permit. Firstly, the vegetable oil chemical composition and the main commercially available vegetable oil precursors are exposed. Concerning vegetable oils-based polyurethanes, research groups first focused on cross-linked systems directly from triglycerides then on thermoplastic ones from fatty acids or fatty acid methyl esters. This manuscript focuses on thermoplastic PUs and underlines the literature about the introduction of hydroxyl groups and isocyanate functions onto triglycerides and fatty acid derivatives. Besides, in a view to the isocyanate/alcohol approach, vegetable oil-based thermoplastic PUs and corresponding diols and diisocyanates are described in details. Mots clés : polyuréthanes, bio-sourcé, huiles végétales, acide gras, diol, di-isocyanate.


Introduction
Various renewable resources (cellulose, hemicellulose, starch, lignin, natural oils, chitosan, etc.) have gathered a lot of attention as potential substitute for fossil resources in the polymer field. Three ways have been studied to obtain renewable polymers. The first and second are respectively the fermentation of the biomass and the chemical degradation and transformation of natural polymers. The third route, which is followed in this manuscript, consists in using the biomass obtained from Nature with or without chemical transformation.
Why choosing vegetable oils as renewable resources? Vegetable oils are among the most promising renewable resources thanks to various advantages. Indeed, vegetable oils present different chemical structures and various reactive sites that can be straightforward chemically modified into a large range of tailor-made monomers with variable functionalities. Moreover, the latter are available and annually renewable, regenerated by photosynthesis year after year.
The inherent biodegradability of vegetable oils is also an attractive characteristic in the growing environmental concerns context.
One question, which returns continuously, is the competition with the feed and food industry.
The scientist's opinions are split on this question. On one hand, some support that conventional crops is only a part of the solution and underline the serious competition between the production of food and feed, with the bio-based products (chemicals and polymers) and the transportation biofuels (bioethanol and biodiesel). They recommend the use of agricultural wastes and the development of both new crops grown on marginal land, and fast-growing vegetative biomass such as grass, wood, etc. [Gallezot 2007a] On the other hand, it has been shown that biomass can be produced in a sufficient volume for industrial utilization without compromising the food supply for the increasing global population.
[Schäfer 2011] Besides, the needs of raw materials for bio-based products will always be very modest compared to the food and feed requirements. ] Indeed, the bio-based and biodegradable plastics rely on only 0.006% of the global agricultural area of 5 billion ha in 2011, and is predicted to be 0.022% in 2016. [Bioplastics] To replace fossil resources in polymer science, different strategies need to be studied at the same time such as the use of wastes (agricultural and others), the use of inedible resources, the improvement of biomass growing and the use of various renewable resources (vegetable oils but also others) to satisfy the requirement of sustainability and to get the expected performances.
Another key issue up to date is the high cost involved in processing renewable resources into chemicals compared with their synthesis from fossil resources. The difference is that processes from fossil resources have been continuously improved during more than one century resulting in a very high degree of technical and cost optimization, whereas methods to obtain chemicals from biomass are comparatively in their infancy. Therefore, wide research and development efforts in biotechnology, chemistry and engineering are required to reduce processing cost by valorizing co-products and by-products and optimizing the inputs (feedstock supply, water management) and outputs (energy and product recovery, treatment of waste). [Gallezot 2007a] The life cycle analysis of the synthesized product is also a crucial tool to take into account in order to validate whether the use of biomass in such or such application is relevant or not.
In the last decades, numerous studies and reviews have emerged using fats and oils for the polymer field. [Schneider 2000, Meier 2011, Schubert 2007, Metzger 2009, Larock 2010, Cramail 2013a, Mecking 2016 Vegetable oils: general data The main constituents of vegetable oils are triglycerides, which are the esterification product of glycerol with three fatty acids. The fatty acids account for 95% of the total weight of triglycerides. Each vegetable oil contains different triglycerides with various fatty acid chains.
Fatty acids and fatty acid methyl esters can be recovered and purified from vegetable oil respectively by saponification and transesterification with methanol. [Vargas 1998] The common fatty acids that can be obtained vary from 14 to 20 carbons in length, with 0 to 3 double bonds per fatty acid. Soybean and sunflower oils, for instance, consist of largely linoleic acid (C18:2) and oleic acid (C18:1). The fatty acid structure and the compositions of few common oils are given in Figure 1.  The Table 1 summarizes the composition of the average production of the fourth more produced vegetable oil in European country and in France.  Ogunniyi 2006, Vasishtha 1989] and the sebacic acid [Meier 2010, Demirel 2008, as can be illustrated in Scheme 1.
Scheme 1-Castor oil as a platform precursor. [Meier 2010] In addition to fatty acids, glycerol is also a valuable biomolecule; the main utilization of glycerol is the production of glycerol esters, used as emulsifiers, surfactants and lubricants.
Besides, among the various chemicals that can be obtained from glycerol, some of them, which are interesting for PUs preparation, can be listed: the 1,3-propanediol and the glycerol carbonate. [Gallezot 2007b, Morales 2009  Castor oil is an exception in vegetable oils with its naturally occurring hydroxyl groups and thus was studied in priority in PU synthesis. [Meier 2010, Ogunniyi 2006] Different methods (Scheme 2) have been followed such as (1) epoxidation / ring opening of the epoxide, (2) transesterification or amidation, (3) hydroformylation / reduction, (4) ozonolysis / reduction and (5) thiol-ene addition. Nowadays, the commercialization of vegetable oils-based polyols and corresponding PUs is growing.
Scheme 2-Commonly used strategies to introduce OH groups into triglycerides and thus prepare polyols.
[Larock 2011, Mouloungui 2013] Vegetable oil-based polyisocyanates have been studied to a much lesser extent compared to vegetable oil-based polyols. Two strategies have been followed to bring isocyanate groups onto the double bond of the triglycerides; they both use the very expensive silver isocyanate salt AgNCO, prepared from NaOCN, and AgNO3. The silver counter ion in AgNCO ensures that the isocyanate reacts on the nitrogen end, and not on the oxygen end of the group.

[Küsefoǧlu 2008, Küsefoǧlu 2010]
A remarkable large series of papers and reviews investigated the functionalization of triglycerides to get interesting multifunctional PU precursors. The main problem encountered in such cases is the heterogeneity of the monomers due to the distribution of double bonds per triglyceride. This has a negative effect on the mechanical properties and the correlation between the final polymer properties and the monomer structures is more complicated. In light of this, the following part covers the advances made in the synthesis of vegetable oil-based well-defined difunctional PU precursors, collecting the literature data of the different synthetic routes used. Moreover, Corcuera and coll. have used similar polyester diols from dimethyl sebacate with molar mass from 1 320 g.mol -1 to 3 500 g.mol -1 to prepare TPE-Us with HMDI and 1,3propanediol as chain extender. The TPE-Us obtained by a two-step bulk polymerization without catalyst exhibited a molar mass in the range of 52.1 kg.mol -1 to 138.9 kg.mol -1 . The authors have studied the effect of the soft segment molar mass on the morphology and the final properties.[Corcuera 2012] A decrease in the polyester diol molar mass leads to a better phase mixing between hard and soft domains, giving higher Tg for the soft segments than the one for the prepolymer. The same approach has also been used to synthesize polyester diols from azelaic acid and 1,9-nonanediol using titanium (IV) butoxide, with molar masses ranging from 500 to 1 500 g.mol -1 . Afterwards, the hydroxyl moieties were introduced on the double bonds by thiol-ene reaction or ring-opening of the epoxide. By using the thiol-ene addition method, asymmetric ester diols have been prepared from oleic and undecenoic acids by esterification with allyl alcohol and thiol-ene reaction with 2-mercaptoethanol [Cádiz 2011] (see Figure 3). Polymerizations with MDI and using tin(II) 2-ethylhexanoate as catalyst, have led to TPUs with molar masses from 50 to 70 kg.mol -1 and dispersity in the range 1.6-1.9. Amorphous (Tg=8°C to 20°C) to semi-crystalline PUs (Tm=124°C) were obtained depending on the fatty derivative used.
These TPUs revealed both good thermal and mechanical properties as well as no cytotoxic response, which make them possible candidates for biomedical purposes.

Saturated and Unsaturated diols
By taking advantage of the prominent metathesis reaction, Narine and coll. have synthesized 1,18-octadec-9-endiol, a long chain diol from oleic acid, by self-metathesis followed by reduction to the diol (Y=58%).[Narine 2010a] Alternatively, a saturated shorter terminal diol, 1,9-nonanediol, has been synthesized from oleic acid with a purity of 99% (Y=72%). These diols were reacted with different di-isocyanates namely HMDI and the fatty acid-based HPMDI to give TPUs. The properties of such aliphatic TPUs will be described in the fatty acid-based diisocyanate section. Our group reported the synthesis of fatty acid-based diisocyanates using the Curtius rearrangement through acyl hydrazide fatty acid-based derivatives without the use of harmful sodium azide (Scheme 5). [Cramail 2013b] Diesters were first synthesized and then reacted with hydrazine hydrate to form diacyl hydrazides in quantitative yields. Afterwards, these diacyl hydrazides were converted into diacyl azides then into diisocyanates via the Curtius rearrangement. A series of partially and fully vegetable oil-based TPUs were synthesized and a large range of thermo-mechanical properties were achieved. Relatively good thermal behaviors were observed with decomposition temperatures at 5 wt% loss from 230°C to 280°C. For some PUs, a close resemblance to HDPE was obtained in terms of solubility and thermal transitions with melting points close to 145°C.
Scheme 5-Fatty acid-based diisocyanates using hydrazine hydrate. [Cramail 2013b] In the Scheme 6, the different routes to isocyanates or equivalents are summarized.
Scheme 6-Summary of the different routes to fatty acid based-isocyanates or equivalents via the Curtius rearrangement.

Conclusion
Vegetable oils represent an interesting substitute to fossil resources. This manuscript highlights the tremendous research efforts that have been made and are still currently made on the synthesis of fatty acid-based polyurethanes following the isocyanate / alcohol route with a focus on thermoplastic PUs. In most cases, the use of vegetable oil derivatives in thermoplastic polyurethanes leads to the preparation of low glass transition temperature materials due to the inherent aliphatic structure and the presence of dangling alkyl chains.