Lighter, stronger, and best of all, cheaper — these are the basic requirements for the development of new high-tech aircraft construction materials. Following the success of the composite material GLARE, Delft University of Technology is again scoring with the development of a new composite material. Polymer technologists at the Department of Chemical Technology have discovered that the combination of nanoparticles with Nylon (polyamide 6) forms the ideal matrix for fibre-reinforced composites. The addition of nanometre-size clay particles (platelets) can make the material up to five times stiffer while enabling it to be used at much higher temperatures as well.
Compression strength, up to now the Achilles’ heel of standard fibre-reinforced composites, can be improved by as much as 40 by adding nanoclay. Until now polyamide’s susceptibility to moisture restricted its application in composites, but thanks to the use of nanoparticles a whole new range of applications emerges. The first steps have already been taken in cooperation with the composites group of the Faculty of Aerospace Engineering. Soon the Eaglet, a test aircraft used by the faculty, will make its first test flight fitted with a tail fin rudder made of fibre-reinforced nanocomposite.
Eaglet, a Chilean aircraft redesigned in collaboration with TU Delft, originally used materials such as glass fi bre and epoxy resin, a thermosetting material. At the TU Delft Aerospace Engineering faculty, a prototype of the tail fi n rudder has been built using a fi bre-reinforced thermoplastic polymer. The plan is to redesign the component in 2006 using the new Delft nanocomposite material created by Daniel Vlasveld.
Nanopow(d)er
Clay improves the mechanical properties of fi bre composites
Astrid van de Graaf
SEM image showing woven fi bres. The fi bres are approximately ten micrometres thick.
Test sample of a polyamide-6 fi bre composite. The fi bres will resist large tensile forces.
In fl exure, however, the compression side is the weakest link. The fi bres require the support of the matrix. Nanocomposites can increase the modulus of the matrix, i.e. its rigidity, and so provide improved support for the fi bres.
To distribute the clay platelets better in the polymer, a good interaction between the platelets and the polymer is essential. The proper eff ect is created by the addition of surfactants.
Nanoclay consists of stacks of negatively charged platelets, held tightly together by positively charged ions.
A surfactant (a type of detergent) can be added to the suspension of individual platelets.
The positive end of the surfactant attaches itself to the platelet.
The modifi ed platelets do not interact well with water, but interact perfectly with polymers.
Thanks to the perfect interaction the platelets distribute themselves evenly in the polymer. The stacks of platelets cannot
be broken up by mechanical force.
The stacks of platelets automatically disintegrate in water, because the ions readily dissolve in water.
When molten, PA6 granulate (left) can be mixed with the modifi ed clay platelets (centre) to form a nanocomposite (right).
It all started with a simple idea. Professor Stephen Picken and Professor Sjaak Elmendorp of the Nanostructured Materials section in Delft came up with the idea of mixing a polymer with nanoclay particles, and then use the mixture for the manufacture of fibre-reinforced composites. Changing the matrix properties using nanocomposite technology would provide an elegant solution for improving the mechanical properties of a fibre composite without any need for developing an entirely new matrix polymer.
Picken: “A polymer, i.e. a chain molecule, with nanoscale particles is called a nanocomposite. To create a fibre-reinforced nanocomposite, layers of glass or carbon fibre are alternated with layers of nanocomposite material, and then heated and compressed. However, until recently we knew nothing about the mechanical properties of such composite materials.”
Dr. ir. Harald Bersee, associate professor of Design & Production of Composite Structures at the faculty of Aerospace Engineering, thought the idea of the fibre-reinforced nanocomposite was just the thing.
Bersee: “Our group assesses the practical value of a material. Anyone can develop a fantastic material with very special properties, but if it turns out to be ruinously expensive or impossible to process, it won’t do you much good. One of the primary incentives in aerospace development is cost reduction. Lighter materials could be very interesting, but not at any cost. That is why we look at the whole picture, integrating the material, the design, and the application process. This nanocomposite is what we are looking for.”
Picken: “The people at the Dutch Polymer Institute were very quick to recognise the importance of this conceptual research, and supported it by including and funding the project in the Engineering Plastics portfolio. This has created a solid footing for the next stage, which is development.”
Nanoparticles Picken’s group is looking at two different types of nanoparticles for the nanocomposites, plates and rods. The rods are synthetic nanoparticles made of aluminium oxyhydroxide (Boehmite, see text box). The platelets are Montmorillonite clay platelets approximately 1 nanometre thick and a few hundreds of nanometres in diameter.
“The large surface to thickness ratio makes the material particularly
interesting”, doctorate student Daniel Vlasveld explains. “These relatively large though very thin platelets give the composite its great rigidity, even at low concentrations. The clay platelets are not all the same shape. It looks a bit like a sheet of paper that has been torn up into random small pieces.”
For the past four years Vlasveld has been working on research to investigate the properties of nanocomposites and fibre composites based on polyamide 6 (pa6). pa6, also known as Nylon, is a polymer built from the caprolactam monomer. According to Picken, pa6 is an interesting polymer for composite applications. “It offers good mechanical properties, such as low friction, high resistance to wear, and toughness, and it is cheap. This is why it is often used in fibres, plastic gears, and casings for appliances. It also interacts well with our nanoparticles, because it has hydrophilic groups and can easily form hydrogen bonds. The nanoparticles are hydrophilic and adhere very tightly to the pa6 matrix”, says Picken.
This advantage is also the drawback of pa6, since under normal conditions it will absorb up to 3% of moisture, which reduces its rigidity, restricting its scope for composite application.
Experimental According to the researchers, a nanocomposite is made like this. You throw some pa6 granules and a bag of clay powder into a hot extruder. The rotating extruder screws mix the ingredients, and the resulting nanocomposite leaves the machine in the form of an extruded product or a granulate that can later be processed in a press to produce a clear film. The production process causes the nanoparticles to become aligned in the direction of flow. Knowing how to make a fibre-reinforced composite is nothing new either. You simply stack alternating layers of polymer and fibre fabric, and then you heat and press them together.
The department of Chemical Technology has its own extruder that can be used to manufacture all kinds of nanocomposites.
The modulus (rigidity) of a nanocomposite is much higher than that of the unfi lled polymer,
and remains so even at high temperatures.
Test samples of various nanocomposites as used to measure mechanical properties such as rigidity, strength, and toughness. Pressed fi lm made using nanocomposite
material.
Inside the heated press the fi lms melt and the nanocomposite material is pressed between the fi bres.
TEM image showing a polyamide-6 nanocomposite. The platelets of modifi ed clay,
which are about 100 nanometres long and 1 nanometre thick, can easily pass through the gaps between the fi bres. To create a fi bre composite the layers of woven fi bres are alternated with nanocomposite fi lms.
and concentration of the particles to predict material properties like rigidity, viscosity, and diffusion as a function of the temperature. This enables us to design new nanocomposite materials. The models also give us insight into what causes the different properties, and this in turn enables us to find niches that might otherwise be overlooked.”
Peeling Even so, things haven’t always been simple. Initially, Vlasveld’s polymer nanocomposite nanoclay refused to adhere to the glass fibres. “I could just peel the layers apart afterwards. We used an electron microscope to see what exactly was going on. It turned out that the nanocomposite was too viscous and did not penetrate far enough between the fibres, so the layers didn’t stick to each other.”
Adding nanoclay greatly increases the modulus of the polyamide; adding just a few percent by volume can double the rigidity. Good news for applications, but not for processability, since the nanocomposite’s viscosity also increases substantially, so it will hardly flow. Vlasveld’s electron microscope research also showed that instead of being homogeneously distributed through the polymer, in some cases the clay platelets occurred as aggregates, i.e. particles consisting of several clay platelets. The distribution of the platelets greatly affects the flow properties of the nanocomposite.
Vlasveld: “We then looked at the morphology and the concentration of the clay particles. The best way to think of nanoclay is like a stack of paper or a book. Each page represents a clay platelet. A bag of clay powder is a bit like an enormous bookcase. During the extrusion process the bookcase is overturned, and you hope that not only the books become evenly distributed, but also each separate page.”
Metamorphosis Clay platelets carry a negative charge, but in a clay aggregate they stay together because there are positive ions between the clay platelets. As a result, the clay particle is electrically neutral.
Vlasveld: “In water the clay particles automatically separate into individual platelets, a process called exfoliation. If we then add a positively charged surfactant, which are compounds similar to detergents, a thin layer of surfactant will form around each clay platelet.”
The result of this process is that the clay platelet changes from a hydrophilic particle into a hydrophobic particle. In water the modified clay platelets automatically precipitate to form a powder deposit. Since the surface of the clay platelets in contact with the polymer now consists of the hydrophobic parts of the detergent molecules, this greatly improves the interaction with the polymer. “So when we mix organically modified clay powder with the pa6 granules in the extruder, the platelets exfoliate, and the nanoparticles become homogeneously distributed”, Vlasveld explains. “As we progressed, we found out that the company dsm were also working on this subject. They were investigating an alternative method without a surfactant. They were trying to get the clay platelets to exfoliate by forcibly mixing clay powder, water and pa6 all in one go. It saves a process step, but the resulting nanoparticles distribution is less homogeneous. More of the clay particles remain in the form of small aggregates. This affects the stiffness to some extent, but it greatly improves the flow properties, which is very important for our application in fibre composites.” Vlasveld concludes that the application of a nanocomposite in a fibre composite requires a compromise between rigidity and viscosity.
Compression load Thanks to its long fibre reinforcement, a fibre composite is very good at resisting tensile loads. On the other hand, its lack of compression strength has always been a problem, Picken explains. If you bend a platelet of this composite, the “outside” of the material is subjected to tensile stresses, and the “inside” becomes compressed. The compression causes the fibres to bend to the point where the material becomes irreparably damaged. Vlasveld has measured flexural strengths on his fibre-reinforced nanocomposite that are up to 40% higher than those of the standard fibre-reinforced composite using a pa6 matrix.
Section of a test sample of fi bre composite as used for mechanical tests.
Flexural tests show the eff ects of the matrix (PA-6 or nanocomposite) and temperature on the material’s strength.
With a matrix of nanocomposite material, the strength of the fi bre composite is higher than with a matrix of unfi lled PA-6, a fact that is particularly important at high temperatures or in a humid environment. As a result, the composite can be used at much higher temperatures. The original Eaglet tail plane
rudder had been made using a thermosetting material (glass fi bre-reinforced epoxy resin) covering a foam core. The rudder has now been redesigned using a thermoplastic composite material (carbon /PEI, CETEX by Ten Cate Advanced Composites). Next year, a renewed rudder design will be tested using a glass fi bre reinforced nanocomposite material.
The ribs are attached to the skin using the resistive welding process. Inside the rib a mould is fi tted with a fl exible tube around its perimeter. A strip of wire gauze is then wrapped around the outside of the rib. The whole assembly is then lowered in position inside the skin. Compressed air blown through the tube ensures that the edge of the rib is pressed against the skin. A voltage is then applied to the metal gauze, and the current fl ow causes it to heat up, thus ensuring that the rib and the skin melt together.
The construction mould in which the tailplane rudder is made. First, the outer skin is applied in the V-shaped trough, and then ribs that maintain the rudder’s correct shape are inserted and welded in place.
Wind catchers The improved resistance to compressive loads makes the fibre-nanocomposite combination an interesting prospect for all kinds of applications involving stresses and strains caused by flexure. The list of items includes wind turbine rotor blades. The market for wind turbines is still growing, just like the blades of wind turbines. Bigger is better. Fibre-nanocomposite material would be eminently suitable for constructing 30 to 60 metres long rotor blades, Bersee thinks. These blades must be rigid so as not to flex excessively. They are made by vacuum injection in a mould containing glass-fibre mats, a production technique that used to be suitable only for thermosetting resins such as polyester and epoxy resin. Most thermoplastics, including polyamide 6, tend to be too viscous to readily flow between the fibres. By changing from polyamide 6 to its monomer, caprolactam, vacuum injection of thermoplastic pa6 becomes an option. At the Aerospace faculty, doctorate student Kjelt van Rijswijk is currently researching this method.
Bersee: “Caprolactam melts at 70 °C, at which point it flows like water,
enabling it to penetrate nicely between the fibres. It is possible to predetermine the moment at which you want the polymerisation to start, i.e. when the connecting of the monomers to form long polymers chains will start to occur. This can be anything between ten and thirty minutes. Beyond that point, the mixture reacts within a few minutes. By combining this reactive process with the nanoparticles, the mechanical properties of the matrix can be further improved. The use of thermoplastic composites also has the advantage that the material can be reprocessed, something that is impossible with traditional, thermosetting composites.”
Processing The benefits a pinch of clay brings! The maximum processing temperature, or heat distortion temperature (hdt) as it is known in professional circles, for pa6 is about 80 °C, but the addition of only a few percent of nanoclay will raise it to no less than 160 °C.
“The rigidity of a material depends on the temperature, which is why when a polymer can not perform at a certain temperature, we would normally look for a different and often more expensive polymer with a higher temperature range. This is no longer necessary”, says Vlasveld. pa6 without the nanoclay starts to soften at a temperature of 60 °C, at which the amorphous parts in the polymer become soft. The crystalline parts of pa6 remain present until the melting point of 220 °C is reached. In aerospace engineering, with application temperatures ranging from –50 °C to 80 °C, another polymer, pps (polyphenylene sulfide) is used. This does not start to soften until it reaches 120 °C, and its melting point is 280 °C.
A drawback of polymers like pps with increased temperature ranges is that the processing procedures, e.g. pressing the composite sheets, also have to be carried out at much higher temperatures. A higher softening temperature raises the cost of the process and will always affect the properties of the polymer as a result of degradation. Although the addition of nanoclay to pa6 raises the maximum application temperature, it does not increase the temperature at which the polymer melts and can be processed. In other words, Vlasveld's nanocomposite does not require expensive new production techniques. This is what makes the material so special.
Moisture In aerospace engineering, polyamide has a reputation as a low-grade polymer because it absorbs moisture. However, the clay platelets have yet another positive effect on the properties of the polymer. They substantially slow-down the transport of water and gases through the polymer.
Picken: “Our method enables us to fine-tune such diffusion properties. It dispenses with most of the negative effects of moisture absorption, which causes polyamide to be avoided for many applications. This is an interesting development in many respects; think about packaging materials for example. Nanocomposites are highly suitable for plastic beer bottles, since the nano composite acts as the perfect barrier against oxidation, and prevents loss of flavour.”
Thanks to these properties the pa6 nanocomposite will soon be able to compete with the expensive pps.
For years most aircraft manufacturers have been using panels made of thermoplastic materials. Parts of the Airbus A340-500/600 use panels of CETEX glass and carbon fi bre-reinforced materials made by Ten Cate Advanced Composites.
Nanoscale self organisation
Normally, particles in water measuring less than one thousandth of a millimetre, known as colloidal particles, remain suspended. However, such particles can sometimes spontaneously form aligned structures, liquid crystals, in which chaos changes to order. The phenomenon depends on the shape and concentration of the particles. The manner in which colloidal rods can organise themselves into liquid crystal structures in water has been extensively researched by colloid chemist, Professor Henk Lekkerkerker of Utrecht University mainly using Boehmite. (Lekkerkerker was also Picken’s doctorate supervisor). Picken: “We wanted to see what would happen if we put the Boehmite rods into an organic solvent, and in particular if the solvent is a monomer, caprolactam, which we would then allow to polymerise. What would be the eff ect on the mechanical properties, and would there be any optical eff ects? The existing literature contains very little on the liquid-crystal behaviour or self-organising properties in polymer nanocomposites.”
He shows two tubes containing a fi shing line that he has spent a weekend cutting up into short rod-like particles with two diff erent lengths. When he shakes the tubes, the diff erence immediately becomes apparent. The contents of the tube containing the short rods remains in chaos, whereas the longer rods spontaneously form aligned structures. “The self-aligning properties of rods depend on their aspect ratio of length to diameter, which must be 6 or greater”, Picken says. Lekkerkerker’s Boehmite rods have a length to diameter ratio of 20 or more. They are synthetic rod-shaped nanoparticles with a
Short rods can take on any orientation when suspended in a liquid, but long rods tend to become aligned, a phenomenon known as liquid-crystalline behaviour.
As the ratio between the largest and the smallest dimension (axis ratio) increases, the particles contribute more eff ectively to the rigidity of the material. This applies to platelets as well as to fi bres. The platelets reinforce in two directions, whereas the rods contribute to the rigidity in one direction only.
thermoplastic composites produced by Ten Cate Advanced Composites. The Netherlands is the only supplier of thermoplastic composites with continuous fibres for the aerospace industry. Fokker, Airbus, and Boeing come to Delft to learn about new materials.”
Test flight For the next stage of research on the fibre reinforced nanocomposite, Bersee has got himself a brand new extruder. He has got his plans clearly laid out.
“We will start by demonstrating the technology on a small aircraft. If it can be used successfully, it can also be used on larger aircraft.”
For test purposes, the Aerospace Engineering faculty has its own Eaglet test plane; a two-seater designed by a Chilean aircraft manufacturer and built using only plastics and glass-fibre. The prototype was acquired by the faculty. “We keep the Eaglet especially for testing new experimental materials that have not yet made it to the market.”
Bersee has had the aircraft's rudder reconstructed using only thermoplastic composites. “Thermoplastic composites are normally used only in secondary components of an aircraft, so if they break, you can still land the aircraft safely. This is the first time the material has been used in a primary component, the tail fin rudder. The next step, next year, will be to replace the composite with a fibre-reinforced pa6 nanocomposite.”
The project has also been presented at Airbus Industries. Although very interested, the European aircraft manufacturer did not immediately jump at the chance. Bersee will continue with Picken's group, using the results obtained by Vlasveld. The Governing Body of Delft University of Technology has approved the project and promised financial backing for the next two years. Ten Cate Advanced Composites and the duwind\ecn\wmc consortium are also supporting the project. All we are waiting for now is the composite's maiden flight.
For further information please contact Prof. Dr Stephen Picken, phone +31 (0)15 278 6946, e-mail s.j.picken@tnw.tudelft.nl, or Dr Ir. Daniel Vlasveld, phone +31 (0)15 278 8013, e-mail d.p.n.vlasveld@tnw.tudelft.nl, or Dr Ir. Harald Bersee, phone +31 (0)15 278 8175, e-mail h.e.n.bersee@lr.tudelft.nl.
Re la ti v e non -aligne d modulus
Length to width ratio Platelets
Fibres
(Composite image by Ten Cate Advanced Products)
diameter of about 10 nanometres and a length of about 200 to 300 nanometres. “In water, Boehmite already starts to
align at a concentration of 1 percent by volume. In caprolactam, a monomer for PA6, liquid-crystalline aligment of the Boehmite occurs at 3%. This is because of the diff erent properties of the liquid organic monomer. The monomer forms a coating around the rod. This reduces the aspect ratio, and consequently, its propensity for self-alignment”, says Picken’s doctorate student, Ceren
Özdilek, who went to Utrecht to learn how to prepare Boehmite rods.
“We have also developed various ways of modifying the Boehmite rods, so we can manipulate the interaction with the polymer.”
To keep the alignment properties during the polymerisation process, a concentration of at least 7% Boehmite is required. Anything less, and the aligned rods are pushed apart by the growing polymer chains. If the particle concentration is high enough, the polymer chains will also become aligned when the material cools.
Özdilek: “The rods act rather like a mould or template which causes the polymer chains to align themselves in the same direction. The polymer alignment can be readily demonstrated by means of polarisation microscopy and X-ray diff raction techniques.” Clearly the aligned rods and polymer chains improve the mechanical properties. A sheet of the nanocomposite becomes much more rigid along the axis along which the rods and polymers are arranged. This could be an interesting application for materials that need to off er higher tensile strength and stiff ness in one direction.
“It is all early days yet. We have only just completed the small-scale testing of the material. We still need to optimise the material properties and production method”, Picken comments.
