Performance and Processing Enhancements of Aromatic Polyurea Elastomer Systems Prepared from High 2 , 4 ’-MDI Isocyanates

Polyurea elastomer systems have been formulated to produce high performance, rapid curing coatings and caulks that can be applied in adverse climates that would normally inhibit the use of other conventional chemistries. It is the inherently rapid reaction rate of isocyanates and amines that allows for the unique gel and cure rates that enables the consistent application of polyurea elastomers over a broad temperature and humidity range. These fast reaction rates, though favorable to the experienced applicator, can be an initial challenge to those who are new to this technology. In an effort to reduce the reaction rates of these polymer systems, sterically hindered amines and isocyanates have been examined. Recent studies have revealed that aromatic isocyanates with a high 2,4’-MDI isomer content do impart steric disruptions that effectively reduce the reaction rates of typical polyurea spray coatings and dual-syringe applied polyurea caulks. The longer open times of these systems allow for reductions in processing pressures and temperatures of the plural component spray machines typically used for polyurea spray applications. The extended processing time also leads to elastomers with improved flow, leveling, and adhesion to a variety of substrates. In addition, significant performance enhancements of tensile strength, tear strength, elongation, and modulus of the resulting cured elastomers have been observed by using these high 2,4’-MDI isomers versus similar polyurea systems that utilized isocyanates with lower 2,4’-MDI isomer content. This presentation illustrates the physical properties and processing enhancements of polyurea spray coatings and dual-syringe applied polyurea caulk products that are achieved using these new, high 2,4’-MDI products. INTRODUCTION Polyurea spray technology continues to rapidly grow and is becoming increasingly visible due to its specification and use in several new, highprofile applications. The Boston Tunnel Project, the Inchon Airport in Korea, and most recently, the San Mateo Bridge in California have all specified polyurea spray elastomers in large, coating aspects of these projects. Though polyurea spray coating technology is only ten years old, it already has gained global confidence and approvals as a high performance coating system. The many successful case histories and the truly unique performance and processing characteristics of polyurea have made it a technology that can compete in niche areas once exclusively held by the more mature coating systems. A continued area of interest lies in the extension of gel and tack-free times of aromatic polyurea spray systems. The technology developed in the early to mid 1990’s entails very rapid reactions (3 second gel times) and can be quite challenging to those who are inexperienced in handling polyurea systems. The challenge of extending gel times without sacrificing the coating performance is once again being addressed by the recognized leaders in polyurea spray technology. Huntsman Polyurethanes has recently introduced several new isocyanate prepolymers that focus on the issue of system reactivity. These new products contain varying levels of the 2,4’-MDI isomer, which introduces steric kinks into the polymer matrix when blended with the standard 4,4’-MDI isomer. The 2,4’-MDI isomer effectively reduces the reactivity of polyurea elastomers prepared via the reaction of primary and/or secondary amines with isocyanates. Primeaux reported that increased levels of the 2,4’-MDI isomer extended the gel-time of aromatic polyurea systems and provided improvements in the performance of the cured polyurea elastomers. Broekaert and Pille-Wolf reported similar observations upon investigation of the influence of isocyanate isomer composition. A second aspect of their study probed the influence of isocyanate functionality on gel time and performance of polyurea spray systems. Their work examined the performance of polyurea elastomers after typical laboratory cure schedules. As an extension of that earlier work, long term cure schedules of systems utilizing higher 2,4’MDI products were undertaken. To better understand the effect of the 2,4’-MDI isomer content on performance, a study using new, commercially-available isocyanate pre-polymers with varying 2,4’-MDI isomer content was conducted. This paper introduces these new aromatic isocyanate pre-polymers and examines the effect that the varying isomer content imparts upon the performance properties of the cured polyurea elastomer products.