VIP-Visions in Plastics - Nacelles and rotor blades on wind turbines, outer shells of aircraft, patient examination tables - all these technical components must withstand high loads while using a minimum of material. Within the scope of the Carbon Nanotube Innovation Alliance, a research initiative in the field of nano-engineering, the Carbo Air Project partners perform research on fiberglass and carbon fiber reinforced plastics (GRPs and CFRPs), with matrix components reinforced with carbon nanotubes (CNTs).

The development to continuously increase nominal performance in wind power stations requires larger wind turbines and technical components. In the field of aeronautics, aircraft manufacturers are searching for alternatives for aluminum materials primarily used to date, because lowering weight while retaining the same stability makes it possible to construct lighter aircraft with greater energy efficiency. In medical technology, by contrast, the objective is to reduce the amount of material used in examination tables, allowing x-ray examinations to be performed at lower radiation levels. GRPs and CFRPs containing CNTs can contribute to these highly varying requirements.

CNT-reinforced GRP for Nacelle Paneling
Within the scope of Carbo Air, fiberglass mats were saturated with vinyl ester and polyester resins containing 0.3 percent fiberglass by weight to study the effects of CNTs on the mechanical properties of nacelle paneling. Manual lamination, the typical production method for nacelle paneling, was used here. For this purpose alternating layers of textile, matting and resin dispersion were placed in a mold by hand, pressed together and cured. Since the higher viscosity of the resin containing CNTs made it more difficult to degas the material thus increasing the number and size of air pores, the process parameters were optimized and ultra-sonic tests performed to ascertain the quality of the test panels. In addition to investigation and development of the technological aspects, a further focal point in the project was to determine the effect of CNT on the mechanic/physical properties of the material. This included many tests to determine the conductivity and thermal properties, the tensile strength, the flexural properties and the impact strength of the CNT-reinforced material in comparison to reference materials not containing CNT. For example, the CNT-reinforced materials showed an increase of up to 10 % in tensile and compressive strength. Moreover, the maximum sag was decreased by as much as 14 %.

CNT-reinforced GRP for Rotor Blades
Moreover within the scope of the Carbo Air studies, CNT-reinforced GRP specimens for rotor blades were subjected to mechanical testing by the Fraunhofer Institute for Wind Energy and System Engineering using a vacuum infusion process. Here the CNT epoxy resin dispersion was infused into the fiber matting by vacuum. Process-related filter effects occurred leading to uneven distribution as well as CNT agglomerates in the material which can decrease the improvement in the mechanical properties resulting from the CNT. Initially the material was subjected to a tensile test. The glass fibers were aligned perpendicular to the direction of tension. As expected, there was no increase in the strength. In terms of the interlaminary shear strength, however, which was tested by means of a compression/tension test, an 11 %increase was noted in comparison to the reference material.

Nano-modified CFRP in Aeronautics
CNTs have also provided a contribution to greater safety and energy efficiency in aeronautics. However the CNT-reinforced laminates will be competitive here only when their fracture strength has been increased by 35 % to 50 % in comparison to the resin systems presently in use. Moreover processing must be simpler with greater reproducibility and higher economy.
Within the scope of Carbo Air, the project partner EADS, is performing research on carbon fiber reinforced plastics (CFRPs) containing CNTs, which were also produced under vacuum. This process allows economical production of even complex structures. A study of the mechanical properties at the EADS testing laboratory showed that the interlaminary fracture strength can presently be increased by 25 % to 30 % by adding CNTs.

Nano-modified CFRP Composite Materials for Medical Technology
In the field of medical technology, CNTs in CFRP examination tables can contribute to decreasing the radiation required through use of less material and thus reduce the radiation dosage while maintaining high mechanical rigidity. For this purpose the Carbo Air project partner Siemans has developed a modified prepreg process. Here an epoxy resin, previously impregnated with 0.5 % CNTs by weight, is applied to the carbon fiber mat. The prepregs were produced by SGL Carbon, Meitingen for demonstration purposes. Mayr Faserverbundtechnik in Rosenheim then produced the component using a sandwich design. The CNT-reinforced layers of CFRP were positioned on top of one another and provided with a hard foam core. At Siemens in Erlangen the specimens were tested for their mechanical properties as well as their permeability for x-rays. Previous tests on CFRP specimen panels with carbon fiber mats showed an increase of 20% in the rigidity. Moreover the permeability for x-rays was not affected by the CNTs. Presently demonstration examination tables are being tested to ascertain the extent to which the thickness of the CFRP laminate can be reduced.
The project results presented show that CNTs can provide a significant contribution to improving materials, especially in the fields of wind power, aeronautics and medical technology. However the examples clearly show that the distribution of the CNTs in the resin as well as in the fiber matting have a decisive influence on the mechanical properties of the component. Specific adjustments and monitoring of the material parameters is not possible presently, so that further research is required and new methods for characterization and quality control are necessary, because CNTs can satisfy the requirements of the market only when the quality can be maintained at a uniformly high level using economical production processes.
Carbo Air Partners
The Carbo Air project teams includes Siemens, EADS Innovation Works, SGL Carbon, Bayer Technology Services, BASF, INVENT, Fassmer, Schuberth, altropol Kunststoff, Mayr Faserverbundtechnik, Fraunhofer Center for Wind Energy and Marine Technology, the Technical University of Hamburg-Harburg and the Technical University of Dresden.