Chemical hybridizations of polyurethanes with fillers

Abstract

materials whose properties are closely tailored by the structure-property relationship. This leads to a broad property spectrum including foam, coating, adhesive, sealant, and elastomer with an annual production over 12 million tons. Recently, functionalization or hybridization with reinforcing materials opened up even a broad spectrum with value added products in market. Among others, waterborne type (WPU) was the first commercialized in late 1980s to reduce the solvent emission. This was done by introducing ionic groups viz. carboxylic or sulfonate groups, and/or nonionic hydrophilic polymer segments such as polyethylene oxide into the PU. When the isocyanate termini of the PU prepolymer (hereafter called IPU) are exposed to water or moisture, unstable carbamic acid is formed. This gives off carbon dioxide to form amine ends which react with the isocyanate termini to make chain extension or cross-linking. Water curable PUs are applied for molding the orthopedic socket and gypsum. IPUs are the key for chemical hybridization. They are directly reacted with the OH groups of many inorganic and organic fillers such as silica, graphene, clay, starch, gelatin, cellulose etc to form chemical hybrids. Among them, hybrids with sustainable raw materials (starch, gelatin, cellulose etc) become bio degradable. Moreover, fillers carrying multiple OH groups provide the hybrids with multifunctional cross-links which give much higher strength and modulus, especially in rubbery state as compared with the conventional cross-links. The enhanced rubbery modulus augmented the elastic recoil with minimum cyclic hysteresis. A second way of modification is done by capping the IPU with hydroxyl acrylates, typically hydroxyethylacrylate, which is generally followed by UV curing to render cross-linking or hybrids with inorganic fillers such as fumed silica. Prior to the incorporation, acrylic functionality such as vinyltrimethoxysilane (VTMS) or allyl isocyanate (DOI: 10.1002/pola. 24473) is introduced into the fillers. Silica acted as multifunctional cross-link as well as reinforcing filler to provide over 99% shape fixity and shape recovery with only 1% (DOI: 10.1039/b922775j). The UV curing of PU or WPU synergistically combined the advantages of WPU (tack-free) and UV cure (fast), which is a great advantage for coating applications. The IPU can directly react with OH groups of fillers to form hybrids (DOI: 10.1039/c1jm10722d). Or, the IPU can be end capped with glycidol to introduce epoxy groups on the termini, which are subsequently reacted with 3-aminopropyltriethoxysilane (APTES) to form APTES extended PU. The ethoxy groups react with OH groups of fillers via sol-gel type reactions to form chemical hybrids (DOI: 10.1039/ b816691a). In both cases, fillers act as multifunctional cross-links and reinforcement. Novel PUs are yet to come with multiple hybridization and functionalization along with non-isocyanate route.