Automotive PLA and
Automotive PLA and carbon nanotubes Nanotechnology for automotive applications Conductivity (S/cm) 1,4 1,2 1 0,8 0,6 0,4 0,2 0 By A. Tielas, V. Ventosinos, M. de Dios Plastic Product / Process Area Engineering & Development Department Galician Automotive Technological Centre (CTAG) Porriño, Spain PLA/CNT (7%) PLA/CNT (7%) Talc (10%) PP/CNT (7%) PP/CNT (7%) Talc (10%) Fig. 1. Conductivity measured by the Van der Pauw method of 15x15x2 mm polylactic acid (PLA) and polypropylene (PP) pieces filled with the same content of CNT. PLA pieces exhibit more than five times the conductivity of PP samples. R impact Impact R t=∞ Fig. 2. Resistance profile in a polymer/CNT sample during an impact. The continuous development of science is making possible the design of new materials with properties that were unthinkable a few years ago. The constant searching for lighter compounds, durable and compatible with the environment, has become one of the main goals of many researches today. In this sense, nanotechnology has quickly revolutionized the whole picture of current design of high added value materials due to the unique properties that those composites exhibit in fields as diverse as electronics, mechanics, optics or magnetism. Carbon nanotubes (CNTs) perfectly illustrate all the benefits that nanotechnology can bring, especially in the manufacture of polymer based nanocomposites. This is due to, among other reasons, their high electrical and thermal conductivity, which are transferred to the polymer, even using relatively small loads of CNTs. Many studies are being carried out to optimize the fabrication of polymer/CNT compounds, especially to improve the dispersion of CNTs within the polymer matrix. The Galician Automotive Technology Centre (CTAG), through its Engineering and Development department in the area of plastic products, seeks to explore and exploit all the inherent advantages of joining together polymer science and nanotechnology. Committed to the need to preserve respect for the environment by using, as far as possible, renewable sourced materials, CTAG currently develops compounds based on polylactic acid (PLA) and CNTs intended for diverse applications. Indeed, one of the most interesting properties of PLA/CNT composites from the standpoint of the practical applicability is their electrical conductivity. It is known that internal CNT networks are formed beyond a given threshold concentration of filler, the point at which a great increase of the electrical conductivity of the material appears. The electrical behaviour of the polymer also largely depends on the degree of alignment and dispersion of CNTs within the polymer matrix. PLA favours the dispersion of CNTs due to its polar character, and, in fact, PLA/CNT compounds exhibit in the order of five times the conductivity of PP/CNT composites (Figure 1). It has been shown that an external factor that can alter the disposition of CNTs, further produces conductivity changes in the sample. This behaviour allows, for example, the detection of impacts, by conductivity measurements, on pieces made of this material. The potential applications are vast, from the dynamic monitoring of structural damage of key parts of a car, to the localizing and counting of impacts on the surface of an airplane fuselage (Figure 2). Based on the same principle, we have developed prototypes of smart pedals that can detect emergency braking situations and activate adequate safety measures in case of an imminent 18 bioplastics MAGAZINE [01/12] Vol. 7
Automotive collision. Drivers react instinctively by contracting their bodies under impact danger situations, and this fact can reduce the braking efficiency just at the very moment prior to a possible collision. The conductivity of a pedal made of polymer/CNT composite depends on the pressure exerted over its surface, thus it is possible to predict risky braking scenarios and enhance the security profile of the entire car (Figure 3). An added advantage of PLA/CNT composites relies on their ability to act as electromagnetic shields. As many parts of an automobile, and generally many everyday electronic devices, have electronic circuits susceptible to emitting radiation, it is necessary to make use of materials of capable EM shielding in order to avoid interferences between them, and also for the provision of a radiation free environment that meets the current electromagnetic emissions legislation for health (e.g. UNE-EN 50083-212007). First results show the suitability of this kind of material for electromagnetic shielding purposes, to a certain extent due to their high conductivity, in the order of 1 S/cm (S=Siemens), which is in the range of semiconductors. Fig. 3. Fully functional prototype of a brake pedal sensitive to the pressure exerted Although the use of PLA offers many advantages, this material still does not meet the requirements of durability and resistance needed in the automotive industry. It remains a challenge to clearly understand the biodegradation mechanisms of PLA/CNT composites. Although there are numerous studies on the influence of the incorporation of CNTs over the degradation kinetics of the material, the role of nano-fillers over the structural stability of the composite is still unclear. Several factors, such as concentration and functionalization of CNTs, or the surface to volume ratio of the sample, have to be taken into account in order to minimize the degradation and maintain the added value of the nano-filled materials; nevertheless, much more effort should be made with the aim of better understanding PLA/CNT interactions. Nanotechnology offers a great variety of compounds allowing not only the enhancement of electric properties of the polymer, but also the optical, magnetic and mechanical ones. In the near future, and even at present, two important challenges must be faced. First, to try to better comprehend the behaviour of nanometric composites in order to control a large range of amazing new properties, and the most important, to take advantage of those properties, keeping in mind the necessity of producing environmental and health friendly materials for a sustainable progress. Although there remains much hard work to find the best way to combine renewably sourced polymers such as PLA with nanoscale structures, is a foregone conclusion that the partnership between polymer science and nanotechnology opens a new era of intelligent materials with astounding properties. www.ctag.com bioplastics MAGAZINE [01/12] Vol. 7 19
