Progress in Study of Non-Isocyanate Polyurethane

- Nov 04, 2017 -

Non-isocyanate polyurethane (NIPU) is a novel kind of polyurethane prepared by reaction of cyclo-carbonates and amines without use of toxic isocyanates . NIPU has attracted increasing attention because of its improvements in porosity , water absorption, and thermal and chemical resistance over conventional polyurethanes.Their potential technological applications include chemical-resistant coating, sealants,foam,etc. In this paper , on the basis of a comprehensive survey of the currently available literature on NIPU , we sumarize recent progress in NIPU,and mainly discuss the syntheses of cyclo-carbonates oligomers , the reaction mechanism , and the preparation and application of different kinds of NIPU .

    Due to its high elasticity , abrasive resistance , and other outstanding properties , polyurethane (PU) has attracted a lot of attention in both the academic and applied areas . However , there still exist some problems in PU preparation and application .For instance , the conventional method for PU elastomer productionis based on the reaction between isocyanates and oligomers with terminal hydroxyl groups (Scheme 1).In this method , the water isolation of the reaction system is essential , because the free isocyanate groups might undergo reaction with the moisture contained in the air or the substrate (Scheme 2) .This situation can be improved by the development of novel segmented poly(urethaneurea) elastomers(TPUU) which has been reported recently by Asplund and Yilgor et al. According to their research , water could be utilized as a positive agent in TPUU preparation.By adding water in the vapor phase continuously,amines can be created in situ to form the hard segments in the structure of TPUU .

   However , in the formation and storage processes of moisture curing PU coatings , the reaction between isocyanate and water is still troulesome . The system must be carefully isolated from water until it is applied to the surface to be coated . Otherwise , the polyisocyanate component and water would undergo an irrever sible reaction , resulting in a hardened unusable product . Additionally , in the curing process of moisture-curing PU adhesive and seal-ants , drawbacks in their appearance and physical properties might be caused by foaming associated with the generation of carbon dioxide gas . The problems mentioned aove can be solved only partly by adding moisture scavenging compounds such as molecular sieves or latent curing agent .

   Conventional PU is unsuitable for some applications like composite matrix materials , because the hydrolytically unstable chemical bonds in polymer structure make it vulnerable to environmental degradation. Besides , the use of isocyanate , which is resulted from an even more toxic predeessor,phosgene,might causeenvironmental hazards.Exposure to isocyanates can result in health effects such as skin-irritation and asthma.Thus , in these days , the study on porous-free and moisture-insensitive polyurethanes becomes an essential issur which needs special concren .

   The work of Groszos etal. is not worthy because it pioneers the field of PU preparation.Through a reaction between cyclo-carbonate and polyamine urea , they provide a method for preparing hydroxyl carbamate wherein some of the amide nitrogen atoms are substitued while others are unsubstitued.This method also provides an inspiration of PU prparation without the use of iso-cyanate.

   After years of development , the research on so-called non-isocyanate polyurethane(NIPU0 has been highlighted.Commonly , NIPU can be prepared from cyclo-carbonate and amine can be formed during the reaction, a linear or network structureo NIPU withintermolecular hydrogen bonds formed among hydroxyl groups at the B-carbon atom of the urethane moiety could be obtained . Except for some mechanical properties comparable with conventional PU ,NIPU usually displays increased chemical resistance and lower per meability as well as improved water absorption and thermal stability . Moreover , NIPU is not sensitive to moisture in the surrounding environment . These properties endow NIPU with numerous potential applications ,such as chemical-resistant coating , sealant , and foam,etc. In this paper , recent progress in NIPU is summarized ,mainly including the syntheses of cyclo-carbonates and their oligomers,the mechanisom for the formation reaction of NIPU,the preparation and application of various types of NIPU , and so on . 

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