Thermoset polymers and composite materials are important in a wide variety of industries, including aerospace, automotive, and indeed any industry that makes use of fiber-reinforced thermoset polymers. Present technologies for manufacturing high-performance thermoset and fiber-reinforced polymer composite (FRPC) parts rely on curing in large, expensive autoclaves or ovens. For example, curing a small section of the Boeing 787’s carbon fiber/epoxy fuselage is estimated to require 350 gigajoules (GJ) of energy during its eight-hour cure cycle, producing more than 80 tons of carbon dioxide. Consequently, there has been much interest in producing these materials with less energy, reducing their costs and environmental impact, and furthering their application in commercial markets.
According to a paper first published in Nature in 2018 by researchers at the Beckman Institute for Advanced Science and Technology, frontal ring-opening metathesis polymerization (FROMP) could hold a key to rapid, energy-efficient polymerization.
More recently, in 2020, Beckman institute researchers improved their FROMP process. The new method enables a wider range of materials with better control over their thermal and mechanical properties.
How Does Frontal Polymerization Work?
Whereas traditional curing uses heat energy added by an oven for materials synthesis, in FROMP the enthalpy of polymerization provides the energy, with no external heat source needed.
In frontal polymerization, a solution of a monomer and a latent initiator is heated locally until the initiator is activated for polymerization of the monomer, producing heat from the polymerization that further drives the reaction. The autoactivation process produces a propagating reaction wave that rapidly transforms the available monomer into polymer.
“By touching what is essentially a soldering iron to one corner of the polymer surface, we can start a cascading chemical-reaction wave that propagates throughout the material,” said aerospace engineering professor and lead author Scott White. “Once triggered, the reaction uses enthalpy, or the internal energy of the polymerization reaction, to push the reaction forward and cure the material, rather than an external energy source.”
What Type of Polymerization?
Polymerization describes the formation of longer molecule chains from monomers. For example, ethylene gas is the monomer that is polymerized upon contact with catalysts, such as titanium chloride, to create the commodity plastic polyethylene. This is polymerization of a thermoplastic.
FROMP works on thermoset plastics. For these materials, polymerization or curing refers to the process by which polymer resins (in most cases) are irreversibly hardened into plastic materials by crosslinking the polymer chains to create a polymer network. Silicone, synthetic rubber, and epoxy resins are examples of thermoset plastics.
What Materials Can Be FROMPed?
Of course, many scientific papers are published that promise new manufacturing techniques, but the methods don’t actually work in practical contexts or with practical materials. According to the 2018 paper, “frontal polymerization has been used to synthesize a variety of polymeric materials, including functionally graded polymers, nanocomposites, hydrogels, sensory materials and FRPC. Most of the materials used in frontal polymerization to date, however, are unsuitable for high-performance applications.”
Following the more recent 2020 research, Nancy Sottos, a Maybelle Leland Swanlund Chair and head of the Department of Materials Science and Engineering—who also leads the Autonomous Materials Systems (AMS) Group at the Beckman Institute for Advanced Science and Technology—said, “Most of the previous research looked at stiffer materials. This paper is the first time frontal polymerization has been used to synthesize a rubbery material. The new technique allows us to have more control and makes materials that have good engineering properties in terms of strength and stiffness.”
The researchers used a mixture of two monomers, 1,5-cyclooctadiene and dicyclopentadiene, to create materials better suited for practical manufacturing applications.
“These materials are chemically similar to what is used in tires,” said Leon Dean, a graduate student in the Sottos Group, which is part of AMS. “Conventionally, the synthesis of rubbers requires an organic solvent, multiple steps, and a lot of energy, which is not environmentally friendly. Our solvent-free manufacturing method speeds up the process and reduces energy consumption.”
FROMP for Shape Memory
By building a shape from layers of two monomers with differing heat expansion properties, the researchers were able to create a shape-memory effect. This effect occurs when a pre-deformed polymer is heated beyond its glass transition temperature, the point at which the polymer changes from a rigid, glassy material to a more rubbery, compliant material. By layering these polymers, sequential changes were achieved across the object as the temperature change occurred.
“We made a layered material in the shape of a hand, where each layer had different amounts of the two monomers and therefore different glass transition temperatures,” said Qiong Wu, a postdoctoral fellow in the Moore Group, which is also part of AMS. “When you heat the polymer above the highest glass transition temperature and then cool it, it forms a fist. As you raise the temperature again, the digits of the fist open sequentially.”
Is FROMP Ready for Prime Time?
It’s the same old story that everybody knows: new lab research promises to revolutionize manufacturing today, but in practice, it just isn’t quite ready. While frontal polymerization is intriguing, it is not yet prepared to leave the lab.
While the Beckman Institute’s research is ongoing, it’s hard to imagine that the precision and repeatability of FROMP are such that it will be a practical replacement for the traditional curing processes used in manufacturing.
“Although we have demonstrated the tunability of several properties over a wide range, it remains a challenge to adjust each property individually,” Wu said.
In addition, researchers note that scaling up—to, for example, part of an aircraft fuselage—is a challenge.
“Most of our work has been done on a lab scale,” Dean said. “However, in larger scale manufacturing, there is a competition between bulk polymerization and frontal polymerization.”
Lastly, according to another paper published in Elsevier in October 2020, “under some conditions, the front experiences instabilities, which do affect the quality of the manufactured composite part.” This work examines and measures further the sources and causes of these instabilities.
“You can save energy and time, but that does not matter if the quality of the final product is substandard,” Sottos noted. “We can increase the speed of manufacturing by triggering the hardening reaction from more than one point, but that needs to be very carefully controlled. Otherwise, the meeting spot of the two reaction waves could form a thermal spike, causing imperfections that could degrade the material over time.”
While FROMP and the concept of polymerization and curing without the need for expensive external heat sources is certainly promising for manufacturing, right now it’s confined to the lab. However, with so much potential to save time and money in real manufacturing applications, it’s sure to be a matter of some interest to manufacturers and materials scientists in the months and years to come.
What do you think about this new polymerization process?