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Less-Polluting Resins for Molding Composite Structures

Fatty acid monomers are substituted for major proportions of styrene.

Liquid resins that are suitable for use in molding composite- material structural panels and that contain and emit smaller amounts of air-polluting volatile organic compounds than do corresponding commercial resins have been invented. In particular, these resins are under study as less-polluting potential substitutes for commercial vinyl ester and unsaturated polyester resins heretofore used in making lightweight composite panels for some military vehicles.

The commercial resins in question contain volatile, polluting reactive diluents (e.g., styrene and methyl methacrylate) that serve to reduce resin viscosities sufficiently to enable liquid molding. Typical styrene contents of the commercial resins range from 40 to 60 weight percent. Some other commercial resins contain as little as 33 weight percent of styrene, but they exhibit unacceptably high viscosities and unacceptably low fracture toughnesses. In contrast, the resins of the present invention contain no more than 25 weight percent of styrene, their viscosities are low enough for liquid molding processes of the vacuum infusion type, and the mechanical properties of polymers and polymer-matrix composites made from them are similar to those of polymers and composites made from commercial vinyl ester resins.

A Methacrylated Fatty Acid molecule is synthesized in a simple addition reaction.
The main difference between a resin according to the invention and a corresponding commercial vinyl ester resin is that most of the styrene in the commercial vinyl ester resin is replaced with monomers based on fatty acids. Of various alternative fatty-acid monomers, methacrylated fatty acid (MFA) monomers have been found to be best for making composite materials. MFA monomers are synthesized in simple addition reactions between the carboxylic acid groups of fatty acid molecules and the epoxide groups of glycidyl methacrylate molecules to form single product molecules (see figure). Typical reaction temperatures range from room temperature to 80 °C and typical synthesis process times are a few hours.

Each MFA monomer molecule contains one terminal polymerizable unsaturation site; hence, MFA monomers act as molecular-chain extenders, as do styrene molecules in vinyl ester resins. MFA monomers are attractive as alternatives to styrene in liquid molding resins for several reasons: they have fairly high molecular weights, are nonvolatile, and are derived from renewable sources (plant oils), and the cost of producing them has been estimated to be only slightly greater than that of styrene. (Plant oils have long been used to make polymers, but the use of them to make fatty acid monomers for use as reactive diluents is new.)

The molecular structures of the fatty acids incorporated into MFA monomers affect the properties of polymers and composites made from the monomers. In general, as the lengths of fatty acid molecules decrease, viscosities of resins decrease and mechanical properties of polymers and composites become more favorable, but cost is also a factor. Among various MFA monomers, methacrylated lauric acid (MLau) monomers represent a balance among these considerations: they have good resin and polymer properties and low cost. Methacrylated octanoic acid (MOct) monomers cost more than do MLau monomers, but have lower viscosity and their polymer properties are less favorable.

In a simplistic approach, one could replace all of the styrene in vinyl ester or unsaturated polyester resins with fatty acidbased monomers. However, the properties of the resulting resins and polymers would be poor, relative to properties of commercial vinyl ester resins and polymers. The approach followed in the present invention — replacing most of the styrene so as to reduce styrene content to between 10 and 25 weight percent — has been found to be a good compromise in that desired resin and polymer properties are retained, while the volatile organic contents of resins, and the corresponding amounts of volatile organic compounds emitted into the atmosphere are reduced by amounts ranging from 50 to 80 percent.

This work was done by John J. La Scala, Theresa Glodek, Caroline Lochner, Philip Myers, Steven Boyd, and James M. Sands of the Army Research Laboratory; Xing Geng and Giuseppe R. Palmese of Drexel University; Ashiq Quabili, Stephen Andersen, and John Gillespie, Jr. of the University of Delaware; Ken Patterson, Frank Bruce, Edward Bartling, Charlie Johnson, and Lawrence Coulter of Hill Air Force Base; Roger Crane of the Naval Surface Warfare Center Carderock; and Michael Starks and Jorge Gomez of the Red River Army Depot.

Topics:
Composite materials Emissions Materials Polymers Resins Volatile organic compounds
This Brief includes a Technical Support Package (TSP).
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Less-Polluting Resins for Molding Composite Structures

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    Defense Tech Briefs Magazine

    This article first appeared in the February, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 1).

    Read more articles from the archives here.

    Overview

    The document titled "Demonstration of Military Composites With Low Hazardous Air Pollutant Content," authored by John J. La Scala and a team of researchers, presents innovative advancements in the field of composite materials used in military applications. It was published as a reprint from the Proceedings of the SAMPE 2007 Conference held in Baltimore, MD, from June 3 to 7, 2007.

    The primary focus of the research is on the development of liquid resins for molding composite structures that significantly reduce emissions of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Traditional vinyl ester (VE) resins, commonly used in composite manufacturing, are known to release high levels of styrene, a harmful VOC. The study explores the potential of replacing styrene with fatty acid-based monomers, which are derived from renewable resources, thus promoting sustainability.

    The researchers highlight that fatty acid monomers are not only cost-effective but also possess low volatility, making them ideal candidates for formulating high-performance composite resins. The resulting resins can contain no more than 25 weight percent styrene, ensuring compliance with environmental regulations while maintaining desirable properties for military applications. These resins exhibit low viscosities suitable for vacuum infusion methods, which are essential for producing high-quality composite structures.

    The document details the validation and demonstration of these innovative resins in various military applications, including components for Army tactical vehicles such as HMMWV (High Mobility Multipurpose Wheeled Vehicle) hoods and helmet hardtops, as well as composite rudders for the Navy. The research underscores the importance of developing materials that not only meet performance standards but also align with environmental sustainability goals.

    In summary, this document presents a significant step forward in the field of military composites by addressing the environmental impact of traditional materials. The findings contribute to the ongoing efforts to create safer, more sustainable manufacturing processes in the defense sector, ultimately benefiting both military operations and environmental health. The research is approved for public release, indicating its relevance and potential application in broader contexts beyond military use.

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