Schematic of surface modification approach for installing branching n-butyl phenyl species onto the carbon fiber surface.
Schematic of surface modification approach for installing branching n-butyl phenyl species onto the carbon fiber surface.

In real-life settings, composite performance can depend on the compatibility of the interface between different components. One of the most challenging composite combinations is carbon fibers and polypropylene.

“[The polypropylene polymer phase] is extremely incompatible with the carbon fibers,” points out Luke C. Henderson of Deakin University in Australia. “We wanted to see if we could enhance this compatibility using a surface modification approach, with the ultimate goal of improving the physical performance of this composite for broader use.”

Polypropylene is one of the most widely used thermoplastics because it is easy to process, cheap to produce, and highly durable. Its mechanical properties can be enhanced with reinforcing materials such as glass or carbon fibers. However, polypropylene’s inert molecular structure makes adhesion to the embedded carbon fibers difficult and an additional component, or co-monomer, is usually added to boost compatibility. This, however, typically comes at a cost in terms of processability, expense, and performance.

Instead, Henderson and colleagues at the DEVCOM Army Research Laboratory take advantage of the conductivity of carbon fibers to modify their surfaces with small molecules using an electrochemical modification approach [Randall et al., Composites Part A 159 (2022) 107001,]. The electrochemical grafting of n-butylphenyl groups onto fiber surfaces and incorporation into polypropylene improves the composite’s toughness by 30-32%, depending on the weight fraction. Composites with surface-modified fibers can withstand larger impacts before failure than those with untreated fibers, without impacting detrimentally on other physical properties.

“Our unique perspective was to make the surface of the fiber more like that of polypropylene – using weaker intermolecular forces – while trying to capture 100% of the potential interaction with the bulk polymer,” explains Henderson.

Since the approach uses covalent bonding to attach the small molecules to the fiber surfaces, it is much more robust because they cannot be removed like a coating. The process is also quick, irreversible, and significantly milder than many treatments used in carbon fiber processing.

“The advantage of this approach is that an analogous electrochemical approach has been used in carbon fiber manufacture for almost 50 years, so our modification strategy can be directly mapped onto existing large-scale infrastructure,” says Henderson.

The team have produced modified fiber composites on a pilot-scale facility and are now working on enhancing other physical properties such as tensile strength and stiffness. The improved composites could prove useful for hydrogen pressure vessels or automotive components subject to minor collisions, where impact-resistance is critical.