Investigation of the curing process of spray polyurea elastomer by FTIR, DSC, and DMA

Abstract

The curing process of a typically formulated spray polyurea elastomer (SPUA) system has been thoroughly investigated with the help of a combined analysis of Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis. The evolvement of the microstructure and mechanical responses of the SPUA system during the curing process have been described in detail. The experimental results indicated that the fast curing property of SPUA, which mainly based on the reaction between the primary amine groups and the isocyanate groups, led to the formation of the unstable hard segments in the interphase. After the tack-free time, the residual isocyanate groups contin-ued to react with the rest of the secondary amine groups of the chain extenders for several hours and caused the increase of the molecular weight of the hard segment in SPUA. Furthermore, during the curing process, the ''disordered'' hydrogen bond formed by one C¼ ¼O and one nearby NH (secondary amine) reconstructed to ''ordered'' bond, which contributed to the increase of the content of the hard segment and ultimately, improved the mechanical properties of the SPUA system greatly.

Understanding the role of isocyanate dilution toward spraying of polyurea

“…The absorbance at 1632–1643 cm − 1 , appearing due to H‐bonding between the carbonyl group (>CO)and the adjacent NH functionalities of urea, commonly referred to as “ordered bonding” also increased with time. For the purpose of quantification, the absorbance intensities associated with these characteristic bands were normalized with respect to the absorbance due to benzene appearing at 1600 cm − 1 , and the results are also presented in Figure . Although the reaction is instantaneous, with solidification occurring in a matter of a few seconds, it was observed that the isocyanate absorbance completely levels off after ∼8 h.…”

“…In view of the fast reaction kinetics, the most important concern associated with the processing of polyurea is the requirement of rapid mixing of reactants; which held back its commercialization till the late 1980s. For all practical applications, polyurea coatings are processed by using reactive spraying where, the isocyanate and amine are stored separately and are made to react in the mixing‐module, just prior to being pushed out of the orifice under pressure . A general schematic is presented in Supporting Information Scheme S1.…”

Emergence of time‐dependent material properties in chain extended polyureas

“…The rapid kinetics associated with amine‐isocyanate reaction led to processing complications, which withheld its commercialization. Subsequently, the development of improved spray equipment made it possible to overcome these technological issues, thereby rendering the process commercially realizable …”

“…Unfortunately, in view of the high reactivity of isocyanate and amine, polyurea coatings tend to build up huge internal stresses during the initial stages of solidification associated with the instantaneous curing process. It is to be noted that although the curing process has been studied using various analytical techniques, the time frame over which these studies have been performed is rather short . The microstructure of polyurea comprises of two distinct domains: “hard” realms formed by hydrogen bonded polar urea linkages (NHCONH) and “soft” domains comprising of long aliphatic polyether chains .…”

“…The phase‐segregated structure of polyurea bestows extremely interesting properties to the polymer, especially enhanced modulus, high extensibility, and resiliency . The formation of hard phase in polyurea, as studied using fluorescence and infrared spectroscopy, has been reported to reach completion within 400 min of reaction . It is to be noted that although the solidification occurs in a matter of seconds, the relaxation of internal stresses mandate much longer periods.…”