This topic was discussed initially, almost 6 years ago. Since that time there has been an effort to get away from using isocyanates for polyurethanes. The quest for sustainable and green polymeric materials continues to be a holy grail. Therefore, nonisocyanate polyurethanes (NIPU) materials are considered the best consideration of being nontoxic and ecofriendly and have created a great deal of attention as equated to the conventional polyurethane (PU) obtained from petroleum resources and isocyanates.
The continuous quest for green and sustainable polymeric material has become a topic of interest. In this context, NIPU materials are considered the best being eco-friendly and nontoxic in nature and have attracted great deal of attention as compared to the PU obtained from petroleum resources and isocyanates. This review article first highlights the importance of NIPU over the conventional PU. The review further discusses the summary of different synthetic approaches for partial green polyurethane (GPU), which was produced using vegetable oil and isocyanates. Further, to remove the shortcomings of isocyanates and to produce complete GPU, the production of NIPU has been discussed in detail. The recent literature on the advancement in NIPU have also discussed in this review along with their properties and applications in the various fields.
New articles highlight the importance of NIPU over the conventional PU. The assessments further discuss the summary of different synthetic approaches for partial green polyurethane (GPU), which was produced using vegetable oil and isocyanates. To remove the shortcomings of isocyanates and to produce complete GPU, the production of NIPU has been deliberated in detail.
In one article, NIPUs based on five-membered cyclic carbonates have emerged as the most promising substitute to replace more toxic, conventional polyurethanes. The low reactivity of five-membered cyclic carbonates has restricted the preparation of one-pot systems because of long curing times. This work, which was completed by Alvaro Gomez-Lopez, Bruno Grignard, Iñigo Calvo, Christophe Detrembleur, and Haritz Sardon of the Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 Donostia-San Sebastián, Spain, focuses on the enhancement and application of these materials as adhesives through the combination of NIPU chemistry and a sol–gel process that allows for curing by atmospheric humidity.
The synthesis of NIPU prepolymers functionalized with (3-aminopropyl) triethoxysilane (TEOS) is demonstrated. Their curing behavior is examined by means of rheological experiments and a lap-shear test, respectively. In spite of the ability of the alkoxysilane to cure under ambient conditions, the results show that the use of catalyst and elevated temperatures increases the curing process and leads to improved adhesion properties. Hence, it is demonstrated that the fastest curing and the best performance are achieved at 100 °C when acetic acid is employed as the catalyst.
Hybrid Coating Technologies (HCT) announced that Industrial Finishes & Systems has entered into a definitive exclusive national distribution agreement. HCT has developed Green Polyurethane™, the first-ever modified hybrid polyurethane (currently used in coatings and paint) manufactured without the use of toxic isocyanates throughout the entire production process. The result is a substitute for conventional polyurethane and epoxies that is less toxic, and may provide reduced application costs. Their product claims to be the first commercially available zero isocyanate hybrid polyurethane coating. The product can be applied in environments that make conventional toxic, isocyanate-based polyurethane coatings difficult to use, such as hospitals and schools.
Considering conventional isocyanates and their use in polyurethanes, an isocyanate has the functional group with the formula R–N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. A di-isocyanate is an isocyanate that has two isocyanate groups. Di-isocyanates are manufactured for reactions with polyols in the production of polyurethanes.
Isocyanates are produced by treating amines with phosgene: RNH2 + COCl2 → RNCO + 2 HCl. Phosgene is a highly hazardous material, and the production of isocyanates requires special precautions and care.
Isocyanates are reactive toward a variety of nucleophiles including alcohols, amines, and water. Upon treatment with an alcohol, an isocyanate forms a urethane linkage: ROH + R’NCO → ROC(O)N(H)R’ . If a di-isocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes.
In addition, isocyanates react with water (the hydroxyl) to form carbon dioxide: RNCO + H2O → RNH2 + CO2, which is used for the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent.
Chemistry – Isocyanates/Polyisocyanates
Of the roughly 80% of non-CASE applications, the polyisocyanates (PI) utilized are either Methylene Diphenyl Diisocyanate (MDI, CAS 101-68-8 and 9016-87-9) and Toluene Diisocyanate (TDI); 2,4-TDI (CAS: 584-84-9) and 2,6-TDI (CAS: 91-08-7).
The mixture of diisocyanates known as TDI consists of two isomers:
The starting material is methylbenzene (toluene). When it reacts with mixed acid (nitric and sulfuric), two isomers of nitromethylbenzene (NMB) are the main products.
Both TDI and MDI are aromatic compounds, and as such, provide relatively poor resistance to ultraviolet (sun)light, and therefore are not used in coatings applications where sunlight is severe, but rather in uses where UV is not a concern. TDI is mainly used to make flexible polyurethane foam that can be found in a wide range of products, including furniture, bedding, carpet underlay and packaging. TDI is also used in the manufacture of some coatings, sealants, adhesives and elastomers. TDI helps produce lighter automobile seating and headliners, saving weight and making vehicles more energy efficient. MDI is used primarily to make rigid polyurethane foams such as insulation for appliances such as refrigerators, and many other uses. Insulation made with MDI can help save heating and cooling costs, Vehicle parts like dashboards, steering wheels and bumpers are also made of MDI.
The starting materials are phenylamine (aniline) and methanal (formaldehyde) which react together to form a mixture of amines, known as MDA (methylenedianiline). This mixture reacts with carbonyl chloride (phosgene) to produce MDI in a similar way to the manufacture of TDI. MDI contains the diisocyanates isomers: 2, 4’-MDI, 2,2’-MDI and 4,4’-MDI. The term MDI refers to the mixture of the three isomers in Figure 3. They can be separated by distillation.
Less widely used, but still important, are the aliphatic diisocyanates, including hexamethylene diisocyanate (HDI), hydrogenated MDI (H12MDI), and isophorone diisocyanate (IPDI). However, even in coatings, only 3-5% utilize aliphatic PI, while the remaining 95-97% employ aromatics. HDI, H12MDI and IPDI are most often further reacted to form polyisocyanates, or pre-polymers, which are used to form color-stable polyurethane coatings and elastomers, that can significantly enhance a product’s appearance, lengthen its lifespan and offer high abrasion resistance.
Covestro (f.k.a. Bayer Material Sciences) and Evonik, are the main suppliers of H12MDI, while BASF and Covestro supply IPDI. There are several Asian companies that provide these as well.
There are specialty isocyanates as well. One type contains silane termination, and is chiefly used in the manufacture and production of sealants and elastomers. In addition, there are hydrophilically-modified isocyanates, used primarily to produce TPU for medical applications such as catheters.
Coatings prepared with aliphatic diisocyanates can have excellent resistance to abrasion, as well as superior weathering characteristics, including gloss retention and resistance to yellowing and chalking, as well as lengthening the time between painting cycles. Chemical-resistant coatings made with aliphatic diisocyanates help commercial airliners maintain the durability and resistance needed to withstand harsh atmospheric conditions.
Dependent upon the application and environmental considerations, polyurethanes can be solvent-borne or waterborne. Although great strides have been made in waterborne polyurethane chemistry, they typically do not perform as well as their solvent-borne analogs. A market need and emergent trend is to produce either a 1-part (1K) polyurethane or/and a 2-part (2K) that cross-links without the use of polyisocyanates.
This article was originally posted on January 31, 2021.
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL Solutions or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL Solutions does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL Solutions does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
Thanks for an interesting article. A big step forward over the last twenty years, appears to be that waterborne PU dispersions no longer need N-methyl pyrrolidone
as a co-solvent. I guess that that was needed because – in contrast to acrylic dispersions – it wasn’t possible to achieve dispersion stability by adding carboxyl group functionality. How has it been possible to eliminate the NMP?
best regards
Is TMXDI still available? It is an interesting crosslinker; despite being aromatic, it is light stable. The -NCO group is not directly connected to the aromatic ring.
NMP allows for an easier process in manufacturing PU’s than some alternatives, due to its high solvency and low flammability. However, since regulatory bodies deem it a “bad actor”, other solvents are utilized, including acetone. There are numerous articles written by Covestro, DSM, Alberdingk-Boley and others on the new processes and resultant materials.
I am pretty sure that both Cytec (Solvay) and Chemtura still supply TMXDI. I expect that there may be Asian sources as well.
“The term MDI refers to the mixture of the three isomers in Figure 3”. Where is Figure 3? Is it missing?
Hi Jay, figure 3 is here: https://www.ulprospector.com/knowledge/media/2016/09/pi3.png
Trey is correct, but I think you are asking what the three isomers are: 2,2′-MDI, 2,4′-MDI, and 4,4′-MDI. The 4,4′ isomer is most widely used.
This business is sold from CYTEC to ALLNEX not to Solvay . Chemtura had never produced TMXDI .. If any technical request I would appreciate if you refer to me. Irina.Kobylanska @allnex.com (global responsibilities for TMXDI and TMI ) Thanks and kind regards, Irina