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Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization

Abstract

Thermoset polymers and composite materials are integral to today’s aerospace, automotive, marine and energy industries and will be vital to the next generation of lightweight, energy-efficient structures in these enterprises, owing to their excellent specific stiffness and strength, thermal stability and chemical resistance1,2,3,4,5. The manufacture of high-performance thermoset components requires the monomer to be cured at high temperatures (around 180 °C) for several hours, under a combined external pressure and internal vacuum6. Curing is generally accomplished using large autoclaves or ovens that scale in size with the component. Hence this traditional curing approach is slow, requires a large amount of energy and involves substantial capital investment6,7. Frontal polymerization is a promising alternative curing strategy, in which a self-propagating exothermic reaction wave transforms liquid monomers to fully cured polymers. We report here the frontal polymerization of a high-performance thermoset polymer that allows the rapid fabrication of parts with microscale features, three-dimensional printed structures and carbon-fibre-reinforced polymer composites. Precise control of the polymerization kinetics at both ambient and elevated temperatures allows stable monomer solutions to transform into fully cured polymers within seconds, reducing energy requirements and cure times by several orders of magnitude compared with conventional oven or autoclave curing approaches. The resulting polymer and composite parts possess similar mechanical properties to those cured conventionally. This curing strategy greatly improves the efficiency of manufacturing of high-performance polymers and composites, and is widely applicable to many industries.

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Fig. 1: Overview of the frontal-polymerization concept.
Fig. 2: Advanced manufacturing with FROMP.
Fig. 3: Composite fabrication and modelling.
Fig. 4: Characterization of the FROMP curing approach.

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Acknowledgements

This research was conducted as part of the Center for Excellence for Self-Healing, Regeneration and Structural Remodeling, supported by the United States Air Force Office of Scientific Research through award FA9550-16-1-0017. We thank J. Sung for preparing the micropatterned silicon substrates, T. Ross for sample photography, D. Loudermilk for graphics assistance, and the Beckman Institute for Advanced Science and Technology for use of their facilities and equipment. I.D.R. thanks the US Department of Defense for a National Defense Science and Engineering Graduate Fellowship. L.M.D. thanks the National Science Foundation for a Graduate Research Fellowship.

Reviewer information

Nature thanks J. Pojman and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Authors and Affiliations

Authors

Contributions

S.R.W., J.S.M., N.R.S. and P.H.G. directed the research. J.S.M., S.R.W., N.R.S. and I.D.R. conceived the idea. I.D.R., M.Y., P.J.C., J.E.A., D.G.I., E.M.L. and L.M.D. performed the experiments. E.G. conducted the computational studies. All authors participated in writing the manuscript.

Corresponding author

Correspondence to Scott R. White.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Characterization of DCPD gel and pDCPD products.

a, Heat of reaction, measured by DSC, for the formulation presented in Fig. 4b after ageing the resin at 23 °C for the indicated times. As the curing proceeds, the exothermic peak shifts slightly to lower temperatures and broadens. b, Heat of reaction of pDCPD specimens produced by liquid FROMP, gel FROMP and conventional (oven) cure approaches, indicating fully cured products with a degree of cure of 99.6%, 99.6% and 99.7%, respectively. c, Heat of reaction of the gel before 3D printing and of a cured part after printing, measured by DSC. There is minimal heat of reaction in the printed polymer, indicating a 99.2% degree of cure. d, Rheological profile of the 3D printable gel, showing shear thinning behaviour.

Extended Data Fig. 2 Simulation of FROMP reaction.

a, Schematic representation of the axisymmetric model of FROMP in a glass tube. b, c, Propagation of the reaction front in neat resin. d, Schematic representation of the axisymmetric model of FROMP in the presence of a carbon-fibre tow placed at the centre of the glass tube. e, f, Evolution of the location and profile of the polymerization front in the presence of a carbon-fibre tow.

Extended Data Table 1 Inhibitor concentration and resin incubation time for different manufacturing techniques and the corresponding front temperature and velocity
Extended Data Table 2 Comparison of FRPC panels made with different manufacturing techniques
Extended Data Table 3 Physical and thermal properties of the various components used in computational modelling

Supplementary information

Video 1: Frontal polymerization of an elastic pDCPD gel rolled into a cylinder.

An elastic pDCPD gel is manually shaped into a cylinder and frontally polymerized with a point heat source. The front propagates radially and solidifies the gel into a rigid cylinder within about  3 minutes. The video is presented at 5× speed. The scale bar is 1 cm.

Video 2: Continuous 3D printing and frontal polymerization of a pDCPD helix.

A DCPD gel is printed and simultaneously frontally polymerized. The front initiates where the gel contacts with the heated print bed (70 °C). The front follows behind the nozzle and cures the filament as it is being extruded, forming a freeform helix. The printing takes about 2 minutes. The video is presented at 3× speed. The scale bar is 5 mm.

Video 3: In-plane frontal curing of a 12-ply carbon fibre composite initiated by one resistive heating wire.

FROMP of a 12-ply composite is initiated by powering a single embedded resistive heating wire for 20 seconds. The front propagates from left to right and the part fully cures in about 2 minutes. The video is recorded using a thermal infrared camera and is presented at 4× speed. The scale bar is 5 cm.

Video 4: In-plane frontal curing of a 12-ply carbon fibre composite initiated by two resistive heating wires.

FROMP of a 12-ply composite is initiated by powering two embedded resistive heating wires for 20 seconds. The front propagates from both left and right ends and the part fully cures in about 1 minute. The video is recorded using a thermal infrared camera and is presented at 4× speed. The scale bar is 5 cm.

Video 5: Through-thickness frontal curing of a 12-ply carbon fibre composite initiated by a surface heater.

FROMP of a 12-ply composite is initiated by powering a surface heater from below the layup. The front propagates through the thickness and the cure is complete in about 30 seconds. The video is recorded using a thermal infrared camera and is presented at 4× speed. The scale bar is 5 cm.

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Robertson, I.D., Yourdkhani, M., Centellas, P.J. et al. Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization. Nature 557, 223–227 (2018). https://doi.org/10.1038/s41586-018-0054-x

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