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Development of nanofilled prepreg for aircraft composite structures

Final Report Summary - CNMD (Development of nanofilled prepreg for aircraft composite structures)

Executive summary:

This document represents the final report on the research programme CNMD: Development of nanofilled prepreg for aircraft composite structures, funded by the European Community within the JTI-CS-2009-01-GRA-01-008 framework.

The document provides a summary of the obtained results, highlights limits and lessons learned, envisages future development.

The societal implications of the project, including gender equality actions, ethical issues, efforts to involve other actors and spread awareness as well as the plan for the use and dissemination of foreground are reported.

Project context and objectives:

The introduction of advanced composites materials in modern aircraft primary and secondary structures presents special challenges due to their inherent low electrical conductivity. Although carbon fibres are good conductors, polymeric matrixes are excellent dielectrics, reducing composite structures conductivity.

Among the characteristics of primary structures of a Green Regional Aircraft, it appears essential to assure a relevant efficiency with respect to the requirements for lightning strike protection, potential discharge, electrical grounding and electromagnetic shielding. Lightning strike is a threat to all structures, whether metallic or composites, and requires careful consideration from a certification standpoint. Lightning can induce damage on aircraft structures, as consequence of melting or burning at lightning attachment points, resistive heating, magnetic force effects, acoustic shock, arcing and sparking at joints, and ignition of vapours in fuel tanks (Lightning Strike Protection For Composite Structures, High performance composites, July 2006, http://www.compositesworld.com/articles/lightning-strike-protection-for-composite-structures).

To improve the electric behaviour currently composite materials use add-on solutions such as embedded metal screens or highly metal loaded films for the management of electrical currents.

The implementation of such solutions implies weight penalties and manufacturing complexity due to the addition of non-structural parts, therefore the potential benefit for the use of composite materials cannot be fully exploited.

To obtain a good potential discharge and electrical grounding, it is necessary to achieve a more homogenously electrically conductive CFRP material, in particular in the z-direction, to improve the dissipation of built-up charges. This could be realised by increasing the electrical conductivity of resin matrixes and thus composite structures through-the-thickness conductivity.

Potential solutions are based on the use of carbon nanotubes (CNTs), which were found to be efficient, even at concentration lower than 1 % w / w to enhance epoxy systems and composites electric conductivity.

Iijima's identification of multi-walled carbon nanotubes (MWNTs) in 1991 and single-walled carbon nanotubes (SWNTs) in 1993 (along with Bethune et al.) ignited a firestorm of interest in these remarkable molecules because of their extraordinary properties and potential applications (Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354:56-8, Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993; 363:603-5). Much of the recent interest in nanotube research stems from their unusual geometry, particularly the juxtaposition of molecular and macroscopic length scales in the same ordered structure. This geometry has caused some disagreement on the classification of nanotubes but at the same time is the reason for their intrinsic high electric conductivity of CNTs (Micah J. Green, Natnael Behabtu, Matteo Pasquali, W. Wade Adams, Nanotubes as polymers, Polymer 50 (2009) 4979-4997).

While the inclusion of carbon black into aerospace grade resins is well understood, the introduction of CNTs into RTM resins or prepregs for CFRP components offers some specific challenges which are only insufficiently addressed to date. Even low nanotubes contents significantly increase resin viscosity often resulting in manufacturing and process difficulties, nanofiller aggregation and inhomogeneities through the composite thickness.

The main objective of this project was to develop and optimise the process and manufacturing conditions of a novel nanomodified prepreg based on specifically designed epoxy / nanofiller systems.

The activity was structured in two different work packages (WPs):
- WP1 was focused on the selection of CNTs and resin systems, the evaluation of different dispersion methods and the manufacturing of a new nanofilled carbon fiber reinforced prepreg.
- WP2 was focused on the manufacturing and characterisation of coupons and small panels for testing purpose. A specific testing campaign was carried out to evaluate the benefits coming from the nanomodified composite, include mechanical tests, direct-current (DC) conductivity and electromagnetic shielding efficiency measurements. In addition the effects of a simulated lightning strike event on composite panels were evaluated.

Project results:

Task 1.1: Feasibility study and test plan

A preliminary feasibility study was performed on a CNTs modified Bis-A based epoxy system.

MWCNTs from Nanocyl were selected for their conductivity performance and low cost. Nanocyl 3100 thin CNTs are characterised by a high aspect ratio, an average diameter of 10 nm and an average length of 1.5 - 2.0 µm. They are produced via catalysis carbon vapour deposition (CCVD) and purified to greater than 95 % carbon. The MWCNT modified system was used to manufacture 40 linear meters of approximately 150 gsm resin film. Then four nanomodified panels were manufactured by RFI and a process optimisation phase was performed selecting the best resin film/carbon fabric configuration in order to minimise porosity.

These preliminary activities on the selected CNT modified epoxy system demonstrated the composite manufacturing feasibility and provided useful indications on the resin filming and prepregging process parameters.

Task 1.2: Material selection and dispersion method scale-up

For the purpose of the task objectives three developmental activities were carried in parallel:
(a) resin system reformulation;
(b) replacement of Bis-A epoxy resin with a CEM structural system which is already qualified and widely used in the aerospace market;
(c) CNTs dispersion process optimisation and scale-up;
(d) nanocomposites manufacturing process and scale-up.

Two different product forms were used to pre-screen commercial carbon additives and to evaluate the effect of the dispersion method on CEM systems electrical conductivity:
(a) dispersion of dry MWCNTs in epoxy resin;
(b) MWCNT / epoxy master-batch.

Dispersion of dry MWCNTs in epoxy resin

Two different commercial carbon based products were selected to evaluate the potential of dry additives in enhancing CYCOM 977-20 and CYCOM 977-2 electrical conductivity. Nanotubes were pre-mixed in the epoxy components of the system at room temperature in a glove box and then dispersed according to one the processes described hereafter: ultrasonican and high shear three roll milling.

Ultrasound is an effective solution for de-agglomerating and dispersing fillers at the nano scale into liquids (Yu Zhou, Shan-Tung Tu and Xishan Xie, Fabrication and Mechanical Properties of Carbon Nanotubes / Epoxy Resin Composites Prepared by a Sonication Technique, Key Engineering Materials, Vol. 353-358 (2007), pp. 1374-1377). The sonication process creates alternating high (compression) and low pressure (rarefaction) cycles capable of breaking particle agglomerates and ensuring good dispersion levels in the resin medium. MWCNTs were sonicated in epoxy resin at RT for about 30 minutes to achieve satisfactory dispersion levels.

A three roll mill from Exakt (Germany) was also evaluated to pre-disperse CNTs into epoxy resins. Good dispersion levels were achieved as effect of strong shear forces generated by three horizontally positioned rolls (feed, centre and apron) rotating in opposite directions and at progressively higher speeds.

MWCNT / epoxy master-batch

Different commercially available pre-dispersions of CNTs in epoxy resin were also screened. Available CNTs pre-dispersion were reformulated and the curing agent was then added to the dispersion to manufacture several CNTs modified CYCOM 977-20 and CYCOM 977-2 resin systems.

Nanocomposites manufacturing process, evaluation and scale-up

The resulting resin systems were degassed and cured at 180 degrees of Celsius for 3 hours, to manufacture resin plaques and coupons for testing purposes. A specific electrical testing campaign was carried out to study the influence of different nanofillers and manufacturing process parameters on the resin system properties. A DC conductivity testing was performed on nanomodified samples showing that the percolation threshold could be achieved for MWCNT concentration of 0.1 % w / w. The maximum performance was observed dispersing 1 % w/w (weight/weight) of MWCNT in the one toughened CEM resin system (CYCOM 977-2).

An efficient dispersion method to process high volumes of MWCNTs modified resins systems was developed. Specifically 7.5 kg of 1 % w / w CNTs modified CYCOM 977-2 resin were successfully manufactured (Milestone 1) in the CEM plant of Wrexham (UK). The process scale-up required a fine tuning of the formulation and of the mixing parameters to avoid resin advancement and viscosity increase due to the selected time/temperature/sequence profile. During the optimisation phase several samples were collected, after each mixing step, and characterised (by means of DSC, rheology and SEM) to evaluate the influence of each process parameter on key resin physical properties and the within batch variation. Three different batches of material were produced according to the optimised protocol and conditions to assess the process repeatibility, batch to batch variation and CNTs distribution.

Task 1.3: Tuning of the prepreg manufacturing process

The activity involved the optimisation of the manufacturing process of a unidirectional (UD) tape based on the preferred 1 % CNTs modified CYCOM 977-2 resin system. A hot-melt direct impregnation process was initially selected based on the resin system physical and rheological properties. Fibre tension, nips temperature, nips pressure / gap and line speed were finely adjusted to manufacture a high quality product with excellent impregnation, fibre consolidation and tack levels (Milestone 2). During the process the fibre areal weight (FAW) and the resin content were continuously monitored and recorded to assess the variation within the batch.

Task 1.4: Prepreg manufacturing

Approximately 190 linear metres (56 m2) of CNTs modified unidirectional tape CYCOM 977-2-34%-190-IMS 24K-300 (prepreg material composed by 1% CNTs modified CYCOM 977-2 resin and unidirectional carbon fibres IMS 24K) were successfully manufactured in the CEM Wrexham plant (Milestone 3).

The manufactured unidirectional prepreg was uniform in quality and condition. A visual inspection confirmed the absence of cut or broken fibres, cured resin, unwetted fibres, wrinkles, resin-rich areas, dry or board areas, and visible indication of moisture. Excellent drapability and handle ability levels were also achieved.

Task 2.1: Coupons and small panels manufacturing

In accordance with the objective of this task a series of composite panels and coupons were manufactured to evaluate the impact of nanoadditives on composite processability, mechanical and electric properties. On the basis of the test plan, all the panels for the mechanical (CAI, Gic and Giic), DC conductivity, lightning strike and shielding effectiveness testing campaign were successfully manufactured. The manufacturing protocol and in particular the curing cycle parameters were optimised to produce defect free and high quality nano-modified panels (Milestone 4).

Task 2.2: Composite characterisation

The panels manufactured in the task 2.1 with 1 % w / w CNT modified 977-2-34-194-IMS24-300 UD tape were used for testing purpose. The modified material mechanical and electric results were compared to the unmodified composite values showing a significant improvement of selected mechanical properties (CAI, GIIc) and DC electrical conductivity. In addition the nano-modified material resulted in very high shielding effectiveness values in the 8-12 GHz frequency band.

Moreover in this task, in order to understand the effects of CNTs addition on the basic damage mechanisms of graphite / epoxy laminated composite due to lightning strikes, simulated tests were performed on pristine and CNTs modified graphite /epoxy laminated composite specimens with no lightning protection system.

Some panels were tested to evaluate the lightning strike direct effects in accordance with DO-160F, Section 23, Paragraph 23.4.2 testing procedure. Tests were performed in a representative environment (milestone 5) in Zones 1A and 2A. According to SAE ARP 5414, Zone 1A regions are representative of radome, nacelles and wing tips that are likely to experience initial lightning attachment and first return strokes, while Zone 2A regions are representative of the vast majority of the airframe like the fuselage that is likely to experience subsequent swept strokes, or re-strikes.

For comparison purposes the same tests were performed on unmodified panels. The impacted panels were then fully characterised by C-Scan analysis and micrographic inspection. The analysis suggested that the lightning damage was limited to the first or to the first two plies for CNTs modified panels, whereas the strike determined a damage propagating for 5 - 10 plies of the unmodified composite panels creating multiple delaminations, broken fibres and microcracks throughout the structure.

The CNTs addition was found to increase the DC electrical conductivity, thus resulting in improved dissipation of built-up charges on the surface and in the z-direction of the modified panels.

Task 2.3: Assessment and benefits

The final analysis revealed several benefits of the new developed material.

The mechanical and DC conductivity results showed the benefits of the CNTs addition on composites performance proving the potential of the technology for aircraft primary and secondary structures. A demonstration of the enhanced electrical properties was completed in a representative environment through shielding effectiveness against electromagnetic waves and lightning strike direct effects tests.

Time and costs constraints imposed by the project allowed only the investigation of a single CYCOM product; however Cytec technology might be potentially applied to other selected material systems.

Potential impact:

Socioeconomic impact

The results of this project are aligned with the objectives set by Advisory Council for Aeronautics Research in Europe (ACARE) - the European technology platform for aeronautics and air transport and to be reached in 2020. Generally the introduction of nanocomposites is expected to provide innovative material solutions able to achieve the following 2020 objectives:
- meet society needs for a more efficient, safer and environmentally friendly air transport;
- achieve global leadership for European aeronautics, with a competitive supply chain, including small and medium sized enterprises.

This proposal specifically aims to improve aircrafts safety level by an extensive application of CNT modified systems for lightning strike and systems protection. The project results can potentially boost the use of the CNT technology in aerospace and advanced industrial applications. More in details, from the economic point of view, carbon nanofibres and nanotubes are promising to revolutionise several fields in material science. The majority of current global revenues for CNTs are generated by relatively large-scale manufacturing of bulk materials for applications where electrical conductivity, increased mechanical performance and flame retardancy are primary design drivers. Composites, field emission devices and batteries are the most prominent and commercially viable current applications. Next generation composite materials for marine, automotive and aerospace market will incorporate sensing capabilities and multi-functionality and will lead to greatly increased revenues over the next 3 - 10 years. Further market developments will depend on material availability at reasonable prices. Prices will fall over the next few years as large companies begin to produce commercial-scale volumes of nanotubes. Large multi-nationals such as Arkema, Nanocyl, Bayer and Showa Denko have significantly ramped up production levels; companies in China and Russia are also producing significantly cheaper nanotubes. Ethical and social impacts are harder to define and sort as good or bad compared to health and environmental impacts. The possibility to manufacture aircrafts composite panels with improved electrical properties may provide some benefits in terms of improved safety for the passengers. The new developed material may be used to mitigate the risks associated to a lightning strike event and other externally generated electromagnetic interference (EMI). The new generation of civil aircrafts depends heavily on electronic systems to implement safety-critical functions. Then civil aviation authorities have become increasingly concerned about the potential for EMI to these critical electronic systems. The possibly to use CNTs to improve the lightning strike protection and EMI performance could be a viable alternative to heavier metal based solutions. Moreover the improved mechanical properties could result in reduced weight of the aircraft composite structures, thus resulting in possible increased payload, improved aircraft efficiency and reduced fuel consumption in agreement with some of the requirements of a next generation green regional aircraft. On the other side there is a potential for hazardous effects of nanoparticles on human health. The studies by Warheit et al. (D. B. Warheit et al, Comparative Pulmonary Toxicity Assessment of Single-wall Carbon Nanotubes in Rats, Toxicol. Sci. (2004) 77 (1): 117-125) present the first comparative toxicological assessments on SWCNTs, according to the reference, the potential hazards through inhalation of CNTs have not been sufficiently evaluated. This substance can be stored for decades in the human lungs increasing cancer risk. CNTs could produce reactive oxygen species which are associated with diminishing cellular activities, such as a decrease in the mitochondrial membrane potential etc. During the project execution, CEM has implements a specific safety procedure. Potential risks and exposures were mitigated and reduced to an acceptable level through the use of minimum dry CNTs quantities, reduced handling times and appropriate PPE. It is expected in the next year to have a more comprehensive analysis of the CNT technology and its effect to implement effective measures for risks mitigation and to prevent its negative impact on society.

Dissemination and / or exploitation of the project results

The key results of the project will provide aircraft and component manufacturers the ability to fully exploit nanocomposites benefits. In addition nanomodified materials can also have a high potential for automotive, rail transport and wind turbine blades applications. The project can influence and provide benefits to all aerospace sectors: large commercial transport, business jet, regional jets, helicopters and military aircrafts. The results of the project have already been widely disseminated within the company and at Alenia. Some meeting were attended in Alenia to discuss potential applications of the new developed material. 50 m2 of 1 % w / w CNT modified 977-2-34-194-IMS24-300 UD prepreg were shipped to Alenia for evaluation and a test plan was agreed as follow-up of the activities including lightning strike tests to be performed by Onera. The project has provided important results; some of them will remain confidential, at least until the protection of these results have been implemented, throughout the registration of possible patent applications. The plan for the following dissemination includes the participation to a European Composites Technology conference such as JEC / SAMPE.

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