The demand for composite materials is increasing in the military and commercial sectors because of the need for lowcost and lightweight materials. In many of these applications, vinyl ester resins are used because of their low cost relative to epoxy resins. However, the Environmental Protection Agency recently enacted new emission standards for composite manufacturing and repair that specifically target styrene as a regulated hazardous air pollutant (HAP). Reducing the emissions from composites structures is critical to complying with environmental regulations within the Department of Defense (DoD).
The objective of this project was to develop low-cost composite resin systems suitable for vinyl ester-based composites that decrease concentrations of volatile organic compounds (VOC) including styrene, reducing emissions throughout the composite life cycle.
Researchers from the Army Research Laboratory and Drexel University have developed two solutions for making National Emission Standards for Hazardous Air Pollutants (NESHAP)-compliant resins that also have excellent resin and polymer performance. The fatty acid based vinyl ester/unsaturated polyester (FA-VE/UPE) resin uses fatty acid monomers as a reactive diluent to replace all but about 20 wt% of the styrene HAPs in the VE or UPE resin. The fatty acid monomers are non-volatile and able to copolymerize with vinyl ester, unsaturated polyester, and styrene. The resulting resins and composites have properties similar to those of commercial resins, but have better dimensional stability, lower exotherm, higher toughness, and produce much lower emissions. The bimodal based vinyl ester (BM-VE) resin uses a mixture of low and high molecular weight vinyl ester monomers to reduce resin viscosity and improve fracture performance while using only 28-38 wt% styrene. The resulting polymers and composites have similar performance relative to commercial resins, while also having higher toughness.
These new technologies can attain from 20-78% reduction in emissions over existing resin systems. Both resins have been used to make large composite structures. An M35A3 composite truck hood for Army tactical vehicles and a DDX hat-stiffened frame structure for the Navy were fabricated and successfully demonstrated. The technologies developed also support the 2002 Federal Farm Bill, which mandates that federal agencies establish preferred procurement for bio-based products. The research team has applied for two patents for the new resin systems.
The new resin systems developed under this project are based on environmentally friendly materials, such as soybean oils, that reduce dependence on petroleum resources and reduce emissions to below the target values imposed on the military under the Defense Land Systems for Miscellaneous Equipment NESHAP. Patent pending low VOC and HAP resins for military composite applications include one system with an emissions reduction of 20% and another with a 55-78% reduction in emissions. The new resins also display an increase in fracture toughness and mechanical properties of up to two times over the baselines of commercial resins. The use of these new resins will allow DoD to continue to perform maintenance, repair, and remanufacturing duties on composite parts without the need to incorporate emission control systems, which can cost up to $1 million to install and have recurring maintenance costs. (Project Completed – 2005)