Assessment of research on the substitution of polyurethanes (PU) by non-isocyanate polyurethanes (NIPU) Free

Polyurethanes (PUs) have been heavily attacked over the last 20 years due to their toxicity at three stages of their life, during their synthesis (use of phosgene), their use (toxicity of isocyanates) and their end of life (landfilling or incineration with the release of toxic amines).

The replacement of polyurethanes has given rise to very significant international research and we have returned to the old carbonate-based chemistries to find alternatives as shown in Figure 1.

Here, we will develop more specifically the carbonate-amine chemistry that is undoubtedly the one that has given rise to the greatest developments in research dedicated to non-isocyanate polyurethane (NIPU). The carbonate-amine chemistry leads to polyhydroxyurethanes, as opposed, to PU and leads to molecules possessing a urethane group and a mixture of primary or secondary hydroxyl group.

It is interesting to take stock of this extensive international research to identify options for the future. There are many reviews1–5 on this subject that it is useless to plagiarize but few of them, however, complete they may be, do not give an overall assessment of this chemistry.

To summarize the chemistry of carbonates, two main types of products were studied – five-membered cyclic carbonates (C5) and cyclic carbonates with more than five atoms. For the latter, even if their reactivity is higher than C5,6,7 their synthesis implements more complex chemistries almost as toxic as that used to prepare isocyanates (phosgene or derivatives). Moreover, larger cycles synthesis are more expensive than the chemistry of C5.

C5 carbonates are the most studied compounds8–12 and most likely to constitute an alternative reality, but it must be recognized that they do not meet the requirements of PU materials on at least two levels. First of all, the reaction is very slow at room temperature and in general it is necessary to heat between 70°C and 90°C to have reactions that can be used industrially. Second, the molecular masses of these materials are relatively low because many side reactions limit their value.13 There are other minor flaws that could be mentioned but they are not critical.

Another chemistry14 was proposed by BASF with original C5 carbonates bearing an exocyclic unsaturation.15,16 In this case the reactivity is higher and proved to be efficient at room temperature. However, the synthesis of the carbonate is very difficult and start from expensive alkyne compound and no industrial development has been observed up to now. Finally, Dow Chemical has patented17–20 a special chemistry by the reaction of aldehyde to carbamate21,22 as shown in Figure 2.

It is therefore still a disappointment because the work carried out, often of excellent quality, is enormous. It is also unlikely that we will see, in the future, commercial products such as foams, thermoplastic elastomers, products based on single-component formulations for crosslinkable insulation at room temperature corresponding to a number of high-volume applications and low prices.

Consequently, PUs will remain commercial for a long time to come. In addition, the appearance of new biosourced isocyanates,23 new syntheses of isocyanate24 and new biosourced polyols25 has revived these activities even if they do not resolve the toxicity problems but partially satisfy public opinion.

It should be added, however, that this is not a failure for applications where a slight heating is tolerated during the synthesis of materials, for example, a large part of coatings and composite materials.

To be complete this middle temperature curing chemistry is also in competition with other methods of obtaining urethane groups using ‘cleaner’ molecules than phosgene such as the methods of LOSSEN26 and HOFFMAN27 for which the method of trans-urethanization has been widely used.28–32 However, these methods require quite high temperatures – much higher than in carbonate-amine reactions.

In any case, urethane groups from PU or NIPU remain very interesting groups because they bring remarkable properties in materials to generate strong intermolecular interactions and the presence of additional OH groups in NIPU is an additional element of interest.33 

Finally, C5 carbonate remains the most interesting approach and to make this assessment, we must focus on one last point, the nature or origin of the carbonate.

Generally, these C5 carbonates are prepared by four methods (Figure 3), the epoxides carbonatation reaction (method 1) which remains the main method.

These epoxides can come either from the reactions of alcohols with epichlorohydrins (route 1) or from the oxidation of alkenes (route 2). The other methods of the C5 carbonates preparation, methods 2, 3 and 4, correspond to the carbonatation of vicinal diols34–36 and the catalytic addition of CO₂ to alkenes37–39 and halohydrines,40 respectively.

Figure 3 allows to highlight other aspects which must be taken into accounts as reactivity of cyclocarbonate formed and toxicity of the different synthesis methods.

Indeed, the nature of the epoxy strongly influences the reactivity of the carbonates, glycidyl carbonates being much more reactive than those in an aliphatic or cycloaliphatic chain.41 Concerning the toxicity, the synthesis of glycidyl epoxides presents a toxicity due to the epichlorohydrins (route 1) which are very toxic. The use of catalyst in methods 1 and 3 led to two difficulties, the first the intrinsic toxicity of catalyst and the second their separation. Method 2 appears to be cleanest but does not lead to reactive C5 carbonates.

All these facts clearly show the balance in the choice between the reactivity of cyclocarbonate and the toxicity of the cyclocarbonate synthesis pathway. Thus, the notion of toxicity must be carefully examined and must take into account all the contributions of chemistry and observe the total analysis life cycle of the products.42 

The bottom line is, of course, that PUs are not ‘dead’, whatever the ecological pressures and the toxicological aspect (REACh has listed diisocyanates in Annex 17 for health reasons), for several reasons.

1. NIPUs are no substitute for PUs in practical terms, that is, both in terms of properties but also in terms of processing in specific applications.

2. The ‘riposte’ of PU defenders has been active both in proposing biobased products and in proposing less toxic isocyanates than the traditional phosgene route.

Depending on the application, users will have to choose between the two routes. For the sake of completeness, however, it should be noted that, to our knowledge, the NIPU route has not yet found a manufacturer for either the multi-carbonate precursors or the NIPU polymers needed to implement this chemistry. Even though many patents have already been filed, NIPU materials are not being developed for two main reasons: the lack of knowledge about the longevity of performance obtained with NIPU materials and regulatory issues. Even though regulations are becoming stricter, there are still many exemptions for the use of certain toxic products. In addition, the registration of new products remains tedious and costly. It should also be noted that there are no specific studies on the degradation of these materials. All of this is delaying the emergence of NIPU materials.