Researchers have found a way to create industrially important thermoset polymer and fiber-reinforced polymer composite products at considerably lower expense than has been possible before.
The approach could create products such as shaped composite panels for airplane fuselages using 10 orders of magnitude lower energy than current industrial manufacturing techniques. And the resulting polymers and composites have comparable properties—strength, thermal stability, bending resistance, and chemical resistance—to those made conventionally.
The method could be useful for making a wide variety of polymer and composite products in a range of creative forms—including strong, lightweight shaped materials for the bodies of cars, boats, and planes, some of the largest-volume applications of fiber-reinforced polymer composites.
Manufacturing high-performance thermoset components currently requires autoclaves that cure preshaped monomer resins by heating them under pressure. The size of the autoclave scales with the size of the component, so some are very large indeed. The process is slow and uses an enormous amount of energy, especially for large components.
Aerospace engineers Scott R. White and Philippe H. Geubelle, chemist Jeffrey S. Moore, materials scientist Nancy R. Sottos, and coworkers at the University of Illinois, Urbana-Champaign, developed the new cost-effective method (Nature 2018, DOI: 10.1038/s41586-018-0054-x).
The researchers estimate that conventional curing of a small section of a Boeing 787 fiber-reinforced composite fuselage requires 96,000 kilowatt hours of electrical energy, the amount used by about nine residential homes in one year. They believe their new curing method would lower the energy requirement for the same part to 9.6 milliwatt hours, enough to light a 25-watt incandescent bulb for about 2 seconds.
The team’s first step is to preshape a solution or gel of the monomer dicyclopentadiene (DCPD) or a DCPD-fiber mixture. They then use a heat source to initiate polymerization of the preshaped material. Once initiated, the monomer has sufficient internal energy to polymerize itself into a thermoset product, with no autoclaves or power plants needed.
The process is called frontal polymerization because the reaction moves quickly through the monomer resin or monomer-fiber mixture along a line, or front, like a rapidly moving weather system or military formation. Frontal polymerization of DCPD produces high-performance, crosslinked thermoset polydicyclopentadiene (pDCPD) polymers or polymer composites.
The specific reaction the Illinois researchers use is ruthenium-catalyzed frontal ring-opening metathesis polymerization, or FROMP (shown). Other scientists developed FROMP earlier, but its use has been severely limited by its short “pot life.” For example, an unheated DCPD monomer resin or fiber mixture cures substantially in 30 minutes. In most cases, that’s too fast to preshape the starting material properly before initiating polymerization.
The group’s key contribution is the recent discovery of alkyl phosphite inhibitors that extend the processing window for DCPD monomers from 30 minutes to 30 hours. The inhibitors make it possible to use FROMP to create a range of pDCPD polymer and composite structures.
The researchers used a 3-D printer to create spiral forms and a DCPD molecular model. They fabricated polymer shapes sporting embossed lettering. And they made fiber-reinforced composite panels. The products have properties similar to those of comparable polymers and composites produced industrially and are suitable for high-performance applications. Because initiating heat is the sole energy required, the method uses far less energy than conventional curing. It is also faster and doesn’t require expensive autoclaves.
“The authors have shown how to extend FROMP pot life to useful ranges, and, most importantly, how to make useful materials faster and cheaper than by traditional methods,” says frontal polymerization pioneer John A. Pojman Sr. of Louisiana State University.
White says that he and his coworkers hope to use the technique to “develop entirely new methods to manufacture complex structures. We have filed several U.S. patents related to this research, but it is not yet being commercialized.”