By and , U.S. Naval Research Laboratory
Polyurethane (PU) networks exhibit the outstanding mechanical strength, thermal stability, and solvent resistance that is required for use in high-performance coatings, adhesives, and composites. However, the most common way to form these polymer networks is to use hazardous isocyanate-functional molecules. Moisture-cure urethane silane polymers have recently been used to form hybrid PU networks, thereby mitigating end-user exposure to hazardous isocyanate during application. However, while the applicator is not exposed to isocyanates, they are nonetheless used to synthesize these polymers. High-oleic soybean oil (HOSBO) was chosen for use in hybrid binders to explore potential beneficial properties, mainly enhanced hydrophobicity and reduced viscosity, and as a U.S.-produced renewable chemical, the fatty acid groups offer beneficial properties. For this project, isocyanate-free polyurethane-silane binders were formed by first reacting diethanolamine with HOSBO to form fatty acid diols, followed by transcarbamation reactions to form urethane linkages. The polymers were then end-capped with various secondary aminoalkoxysilanes via subsequent transcarbamation reactions. Reaction of these polymers with atmospheric moisture resulted in crosslinked hybrid networks with siloxane linkages. Surface-free energy (SFE) as well as thermal and mechanical energy were explored.
Introduction
Crosslinked polymer networks are in numerous consumer and industrial products, ranging from coatings for medical devices to foams for seat cushions.1-4 The chemistry for these networks typically involves urethane, urea, ether, and thioether chemistries.5-7 Among these, polyurethane and polyurea networks remain the most widely studied due to their rapid and efficient formation at room temperature, the ability to modify the backbone structure, as well as their excellent thermal and mechanical properties.8,9 The most common way to form polyurethane networks requires the use of hazardous isocyanates.8 However, isocyanate exposure can lead to a myriad of health issues, including skin and eye irritation, symptoms of asthma, and sensitization upon exposure.10,11
A way to synthesize crosslinked networks while avoiding isocyanates is to make networks with siloxanes where an alkoxysilane is used as a crosslinker. These urethane-siloxane polymers also have the advantage of producing crosslinked networks with enhanced properties from the sol-gel region.12-16 For example, silane-terminated polyurethane and poly(urethaneimide) networks demonstrate improved thermal stability, good adhesion to metals, elastomeric properties, and excellent moisture resistance.17-20
In addition to avoiding isocyanates, there is interest in the polyurethane field to shift from petrochemical to renewable biobased raw materials, particularly soybean oil.21,22 Polyurethane networks are not necessarily hydrophobic and often rely upon incorporation of other materials and segments to make hydrophobic material.23 The addition of the fatty acid chains is predicted to enhance hydrophobicity. Other benefits from the fatty acid chain include reduced viscosity and, by using the high oleic oil, reduced yellowing with a high content of chains with a single unsaturated group. Hybrid urethane-siloxane vegetable oilbased polymers have even been formed through isocyanates24-27 and cyclic carbonate chemistry.28-30 The isocyanate-based networks have the same toxicity issue of the nonhybrid polyurethane while the cyclic carbonate-based networks form pendant hydroxyl groups, which may limit any enhancement to hydrophobicity. Rather than use these methods, the ester linkages of the oil can be utilized to incorporate other organic functionalities, such as alcohols and diols, which can be further reacted.31
Herein, a method is provided to form polyurethane-silane networks that circumvents the use of isocyanates and incorporates renewable biobased materials. In this work, a safer acylation reagent, 1,1′-carbonyldiimidazole (CDI), was used to form the urethane functional groups. These acylating groups were used to attach the fatty acid-based diol, which is based on high-oleic soybean oil (HOSBO), to the alkoxysilane crosslinker and to add chain extenders. These compounds were then used as binders to form clear polyurethane-siloxane networks. To our knowledge, there are no reports of polyurethane-siloxane networks based on HOSBO. This method allows for greater control of structure of the binders. Chain extenders have been explored in the binders. Thermal, mechanical, and surface properties of the resulting crosslinked networks were determined using several analytical techniques and compared with those of a control network.
Continue reading in the March-April issue of CoatingsTech