Room temperature vulcanizing (RTV) silicones are the material of choice for designers wanting to improve and increase Department of Defense (DoD) weapon performance. These RTV silicones are used as adhesives, sealants, coatings, heat insulators, and encapsulating materials. In order for RTV silicones to achieve a high level of consistent adhesion to various substrates, a silane primer is applied prior to silicone application. These primers contain 90 to 98 percent volatile organic compound (VOC) solvents which evaporate into the air. The high VOC solvents damage the environment and are hazardous to the health and safety of the workers using them. In addition, the waste primer and its associated applicators are classified as hazardous waste and must be discarded using highlyregulated, time-consuming, laborious, and costly procedures.
This project sought to develop, evaluate, and transition primerless, self-bonding low temperature curable addition cured silicones that eliminate the use of high VOC primers without compromising strength, durability, compatibility, thermal resistance, and long-term stability.
This project was conducted in four phases with the first three focused on the methodical development of adhesion promoting technology using a risk reduction strategy and the fourth on evaluating the developed technology. In the first phase, current addition cured silicones were modified with a bifunctional adhesion promoter compound. In the second phase, less inhibiting adhesion promoters, based on structures defined by molecular modeling, were utilized in an attempt to develop room temperature curing systems. In the third phase, multiple adhesion promoting functionalities were evaluated using guidance from molecular modeling predictions. The fourth phase demonstrated use of the new primerless silicone formulation on an armament system.
This project produced four sealant/adhesive RTV materials that do not require environmentally unfriendly/health hazardous primers for adhesion. PLS100-630 provides primerless adhesion to metals and glass under an elevated temperature cure cycle. PLS200-866 provides primerless adhesion to metals, glass, and thermoset plastics using a room temperature cure cycle. PLS300-866 and PLS300-630A both provide primerless adhesion to select thermoplastics under an elevated temperature cure cycle. All formulations achieved a high level of adhesion with minimal or no surface treatments prior to bonding. A technology demonstration was conducted using PLS200-866 to bond heat shields on the XM 984 Extended Range Mortar System. Simulated and live fire tests were successful, as heat shields remained securely fastened and sealed using the newly developed primerless RTV formulation. This project was completed in FY 2002.
The development of new primerless silicone formulations will reduce the amount of VOCs emitted to the environment by more than 500,000 pounds per year. In addition, cost savings will be realized from reduced hazardous waste handling and disposal costs, reduced inventory management, elimination of the need to comply with and/or possibly obtain waivers, deviations, and fines related to the many regulatory laws, and avoidance of the potential shutdown of operations as a result of non-compliance.