Innovative method improves strength modulus in carbon fibre

Innovative method improves strength and modulus in carbon fibre

11:38 AM, 28th July 2015
Innovative method improves strength and modulus in carbon fibre
Professor Satish Kumar and research engineer M G Kamath examine the precursor and carbon fibre processed at Georgia Tech.

ATLANTA, US: Carbon fibre are stronger and lighter than steel, and composite materials based on carbon-fibre-reinforced polymers are being used in an expanding range of aerospace, automotive, and other applications – including major sections of the Boeing 787 aircraft. It’s widely believed, that carbon-fibre technology has the potential to produce composites at least 10 times stronger than those in use today.

A research team at the Georgia Institute of Technology has developed a novel technique that sets a new milestone for the strength and modulus of carbon fibre. This alternative approach is based on an innovative technique for spinning polyacrylonitrile (PAN), an organic polymer resin used to make carbon fibre.

The work is part of a four-year, $9.8 million project sponsored by the Defense Advanced Research Projects Agency (DARPA) to improve the strength of carbon-fibre materials. The research was reported recently in the journal Carbon.

“By using a gel-spinning technique to process polyacrylonitrile copolymer into carbon fibre, we have developed next-generation carbon fibre that exhibit a combination of strength and modulus not seen previously with the conventional solution-spun method,” said Satish Kumar, a professor in Georgia Tech school of materials science and engineering who leads the project. “In addition, our work shows that the gel-spinning approach provides a pathway for even greater improvements.”

Kumar explained that tensile modulus – a measure of stiffness -- refers to the force needed to stretch a material by a given amount. Tensile strength expresses how much force is required to actually break the material.

In gel spinning, the solution is first converted to a gel; this technique binds polymer chains together and produces robust inter-chain forces that increase tensile strength. Gel spinning also increases directional orientation of fibre, which also augments strength. By contrast, in conventional solution spinning, a process developed more than 60 years ago, PAN co-polymer solution is directly converted to a solid fibre without the intermediate gel state and produces less-robust material.

The gel-spun carbon fiber produced by Kumar’s team was tested at 5.5 to 5.8 gigapascals (GPa) – a measure of ultimate tensile strength – and had a tensile modulus in the 354-375 GPa range. The material was produced on a continuous carbonization line at Georgia Tech that was constructed for this DARPA project.

“This is the highest combination of strength and modulus for any continuous fibre reported to-date. And at short gauge length, fibre tensile strength was measured as high as 12.1 GPa, which is the highest tensile-strength value ever reported for a PAN-based carbon fibre,” Kumar said.

Moreover, Kumar noted, the internal structure of these gel-spun carbon fibre measured at the nanoscale showed fewer imperfections than state-of-the-art commercial carbon fibre, such as IM7.

Kumar and his team convert the gel-spun polymer mix into carbon fibre via a selective treatment process called pyrolysis. This technique eliminates large quantities of hydrogen, oxygen, and nitrogen from the polymer, leaving mostly strength-increasing carbon.

“It’s important to remember that the current performance of solution-spun PAN-based carbon fibre has been achieved after many years of material and process optimization – yet very limited material and process optimization studies have been carried out to date on the gel-spun PAN fibre,” Kumar said. “In the future, we believe that materials and process optimization, enhanced fibre circularity, and increased solution homogeneity will further increase the strength and modulus of the gel-spinning method.”

© Georgia Institute of Technology News

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