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Fiber-reinforced polymeric composites (FRPC) are great engineering materials that are currently used in a wide variety of applications including aerospace, transportation, construction, marine goods and sporting goods. However, these composite structures are still susceptible to failures primarily due to poor interfacial interactions between polymer matrix and the reinforcing fiber. The aim of the CNT-in-FRPC project is to significantly improve key mechanical properties of FRPC structures by enhancing the polymer-fiber interfacial interactions with the aid of newly developed, chemically bi-functional carbon nanotubes, which are incorporated into the polymer-fiber interface using a novel electrospray deposition method.
Thus, specific objectives of the project are:
i) Development of chemically bifunctional carbon nanotube (CNT) structures tailored to covalently bond with both polymer matrix and fiber surface in FRPCs
(ii) Electro-spray deposition of bi-functional CNTs on the fiber surface and fabrication of FRPCs from them
(iii) Establishment of structure-process-property relationships in these next generation CNT containing FRPC materials

For the first objective, first a single step oxidation process was developed for CNTs using a fluidized bed reactor fed with ozone gas. Detailed structural characterizations of oxidized CNTs revealed high level of incorporation of oxygenated species on the CNT surface. Secondly, an electrospray deposition method was developed to uniformly distribute CNTs on glass or carbon fiber mat from a dispersion. Upon the optimization of key process parameters such as the dispersion concentration, type of solvent, spraying speed and distance, first set of FRPCs containing chemically oxidized CNTs at the polymer fiber interface with as low as 0.01 wt% CNT content in the overall composite was obtained and their detailed mechanical testings showed improvements up to 20% in key properties such as the flexural strength and interlaminar shear strength (ILSS) compared to neat composites without CNTs. Thirdly, chemically bi-functional, such that one functional group can covalently attach to the polymer matrix while the other can covalently attach to the fiber surface, MWCNTs were developed and utilized for the same purpose. In addition to their bi-functional nature, it was critical to render them water dispersible, for an environmentally friendly electrospraying process and minimum damage to the fiber surface. Upon the incorporation of these bi-functional MWCNTs into FRPCs in a similar manner at 0.01 wt% content, flexural strength of corresponding composite panels increased almost 35%, which is one of the first examples in the state-of-the-art in terms of the magnitude of property improvement as a function of the nanofiller content.

These studies clearly demonstrates an alternative approach to reinforce FRPC type laminated composite structures. Typical approaches include the incorporation of nanoaparticles into the polymeric resin, which is found to result in property improvements solely from the reinforcement of the polymeric matrix. However, in this novel approach, both the placement of CNTs at the polymer-fiber interface, and a covalent attachment of them to both polymer and fiber is realized.

The implementation of the newly developed electrospray process and chemically bi-functional CNTs to industrial scale composite manufacturing would be critical to enable designing stronger, lighter-weight composite parts and structures especially for aerospace and transportation purposes. In fact, aqueous dispersions of bi-functional MWCNTs are potentially great intermediate products for this purpose. Similarly, glass or carbon fiber fabric decorated with carbon nanomaterials via the electrospray deposition are promising raw materials especially for pre-preg production. In order to achieve these goals in industrial scale, efforts in the near future will focus on increasing the TRL of the project by fabricating prototype composite panels in collaboration with key partners accross Europe.