Prepreg: The Ultimate Composite Material
Prepreg (abbreviation for pre-impregnated) is a high-tech material fabricated by the combination of a resin matrix and a fiber reinforcement material. At this time, they are used in all aerospace programs worldwide, with Boeing 787 and Airbus A350XWB, containing more than 50% by weight of advanced composite components and other new generation high-speed vehicles such as F1 cars and racing boats and sporting tools [1]. By avoiding variable quality conditions resulting due to different laminators and non-consistent mixing processes, prepregs with real performance benefits took over a large share in the composite market.
Currently resins used in the production of prepregs are prevalently thermoset based and the most frequently used resin types are epoxy, phenolic and bismaleimide resins. Thermoset based prepregs are produced with the pre-curing of the impregnated reinforcement fibers at suitable temperatures. This action is also known as gellation where polymeric matrix transforms from A-stage to B stage. In the prepreg production the thermoset resin is in liquid form and the fiber matrix is “impregnated” within the resin. Upon removal of the excess resin, the prepreg is partially cured and the resin solidifies yet stays still tacky. At this step the prepreg is called as the “B-stage”. The full polymerization is achieved by heating this B-stage prepreg in an oven or autoclave by the end composite manufacturer. The full polymerization is achieved by heating this prepreg material in an oven or autoclave by the end composite manufacturer. This action causes the cross-linking of the polymer chains and results in the formation of a solid material, which is durable, temperature resistant, very rigid and extremely lightweight [1-2].
The condition of the end use of the prepregs is critical and must be well estimated for producing the right type of the material. There are two sorts of reinforcement in a prepreg: unidirectional or fabric (woven or multiaxial). The resin choice is also very critical since it is the resin constituent of the prepreg that supports the fibers by bonding them together, transfer any applied loads to the fiber and keeps the fibers stable. Moreover, it is the matrix system that determines the environmental resistance and the maximum service temperature of the prepreg [3,4].
The fibers used for prepreg production are mainly glass, carbon and aramide but it is possible to find unusual ones such as boron, quartz and ceramic and combinations of those called hybrid fibers for more specific application purposes [5]. Although the resin choice is very important, it is the volume ratio of the fibers, which determines the performance [6]. The more fibers mean the higher is the performance of the material. At this point, the benefits of the prepreg over conventional composites are more obvious since prepreg production provides a way to control and optimize precisely the fiber volume fraction and hence allows tuning the final product’s properties as desired. Moreover, having the ability to strictly control the resin content and its dispersion in the prepreg production removes the risk of variation of the fiber content and the formation of resin rich areas, which are the problems both encountered in other production techniques and seriously affecting the material performance.
Prepreg: the History & Uses
The first prepreg machine was developed in the late 1960’s to support the production of carbon fiber tape for the manufacture of composite structures on a U.S. National Aeronautics and Space Administration (NASA) lunar lander. At that time, resin systems were, for the most part, difficult to mix, catalyze, process consistently and manage during production [7].
In the early 1980’s prepregs were considered specialty materials, accounting for around 5% of an aircraft design and used only for non-critical secondary structures. Into the late 90’s and early 2000 new resins, materials and technological processes with more refined controls over the production were developed. Big material manufacturers such as Hexcel, Gurit and Toray provide prepregs in large scale since 2000’s [7]. Since 2002, according to international reports the global prepreg shipments has grown more than 10% per year [8].
Following aircraft builders, marine constructors have also started using prepregs for premium yachts and top performance catamarans needing structures that are exceptionally light, stiff and strong [9].
Various other applications from fishing rods to airfoils, from wind energy to sporting tools of prepregs fabrics are seen more and more frequently in recent years. More recent applications benefitting from prepreg include subsea tubes for oil and gas exploitation and high-pressure vessels. This growth in the use of prepreg composites over metals has been driven by higher strength to weight performance, better fatigue strength and potential to offer greater freedom of design. Also, big electronic companies like Samsung and Apple fabricate the circuit boards of their products using prepreg materials. In addition, most of the parts used in racing or sports cars, like in F1 vehicles, have been fabricated using prepreg fabrics. All things considered, the variety of given application areas reveals the fact that the prepreg materials will become the steel of 21th century.
Prepreg: Advantages
Prepregs have several advantages over traditional hand layout composites due to their structure. First they have high specific modulus and strength meaning maximum strength properties. The precise resin content of prepregs ranging around 35 % is ideal for maximized curing properties whereas with typical vacuum bagged hand laminates there always is excess resin (with resin contents up to 50%). This excess resin causes brittleness, reducing overall mechanical properties. Moreover, having the ability to design the fiber orientation according to the direction of principal stress and optimizing laminate design maximizes material performance according to the demands of the end user.
When compared to hand lamination techniques, which can easily cause resin rich areas or dry spots, prepregs offer uniformity and repeatability. The production process is meticulously controlled throughout the prepreg production line by monitoring resin content, production speed and prepreg thickness leaving only a small margin for error [2,3,5,8,14]. The prepreg production is also a “clean” process since the draining of the excess resin during the curing process eliminates the messiness (cups of resin, dirty rollers, drips, etc) of the hand layup and it operates at lower curing temperatures saving energy.
Prepregs offers also better visual cosmetics, there are no air bubbles forming inside (low void content), the surface is smooth, it has very low coefficient of thermal expansion, offers good environmental and corrosion resistance and good fatigue resistance.
Prepreg: Market Overview
According to the analyses made by the international research organizations, the global prepreg shipment increased to 57k tons in 2007, from 25k in 2002, which resulted in a sales revenue of 2.2 billion USD. Between 2002 and 2007, the prepreg market reached an annual growth rate of 10%, as a result of high demands from stable and fast-growing industries like wind energy, aviation and marine industries. Despite the negative effects of the global economic crisis in 2008 and 2009, the prepreg market recovered fast and reached a market value of 2.6 billion USD at the end of 2010. It has been forecasted that, as a result, an annual growth rate of 14% in the market, the sales revenue of 3.1 billion USD in 2011 will be increased to 4.3 billion USD in 2015 and to 7.2 billion USD (equivalent of total actual shipment of ~150k tons) in 2017. Figure 1 summarizes the prepreg market projections throughout years. In the light of these projections, companies with prepreg technology have been increasing the capacities of their production lines and making big investments in order to install new production lines.
In the world, Hexcel and Cytec are the biggest suppliers, which can provide almost any prepreg related demand from various industries, like energy, construction, automotive and especially aviation and aerospace industries. For instance, according to the 4 billion USD worth agreement between Hexcel and Airbus in 2009, Hexcel will produce the necessary prepreg fabrics until 2025, which will be used in manufacturing of A350 XWB planes. Hexcel managed to reach a sales revenue of 412.3 million USD in the 3rd quarter of 2013 with an annual growth rate of 5.3%. The best selling products of the company includes epoxy, phenolic, BMI and cyanate based prepreg fabrics with carbon, glass, aramid fibres or hybrid fibres of those.
Cytec agreed to produce the necessary prepreg materials to be used in F35 fighter jets between 2016 and 2025, according to the agreement worth 1.1 billion USD with Lockheed Martin. The company, which can produce a variety of thermoset and thermoplastic resin based prepreg materials for aerospace and different industrial sectors, Toray Industries, became one of the most important prepreg suppliers in the aviation and aerospace industries after signing an agreement worth 6 billion USD with Boeing regarding the production of the 787 Dreamliner for 16 years. In 2014, they achieved a net sales revenue of 225 million USD from its carbon fiber and prepreg products.
Gurit AG is the biggest prepreg supplier for marine and energy industries. It has the biggest volume in production and sales of unidirectional prepregs, which are mainly used in windmill blades and bodies. The Gurit Composite Materials had a net sales revenue of 240 million USD in 2013.
Prepreg: Market in Turkey
DowAksa, one of the leading carbon fiber manufacturers in Turkey is targeting industrial applications of the carbon fiber industry. AKSA signed in 2011 a MOU with Dow Chemicals to form a joint venture to manufacture and globally commercialize carbon fiber and its derivatives [10] .
DowAksa is currently realizing the IPEK project (development of carbon fiber reinforced thermoset resin based prepregs for aviation industries) together with Turkish Aerospace Industries and Undersecretariat for Defence Industries [11].
Apart from this, there are also several other premature attempts by both large and small companies to enter the prepreg market in Turkey.
Nanokomp: Turkey’s Leading Prepreg Manufacturer
Nanokomp Advanced Materials Limited Company is the pioneer of the prepreg production in Turkey. Nanokomp is an R&D company located in Istanbul, Turkey working on providing solutions in order to meet the advanced materials demand of national and global industries. The SME was founded by the support of Scientific and Technological Research Council of Turkey (TUBITAK). Nanokomp is focused on production of thermoset based prepreg tapes as well as developing new generation prepreg production lines. In addition, Nanokomp also provides consultancy services for the application of nanotechnology on engineering materials.
Nanokomp Advanced Materials aims to be the national and regional leader on commercializing composite raw materials in order to meet the large un-met demand in strategically important industries such as defence, aerospace, marine, energy and construction.
Nanokomp’s prepreg production facilities are located in Istanbul and have a daily production capacity of 300 meters for carbon-epoxy, aramide-phenolic and silica-phenolic prepreg tapes. In Fig.2, production of carbon-epoxy prepreg tapes is shown.
Nanokomp has signed a MOU in April 2014 with FGV-CNS a joint venture from Cambridge Nanosystems, a spin-off from Cambridge University, and Felda Global Venture, the biggest oil palm producer in the world. Since then, Nanokomp is the unique supplier of the highest quality available Carbon Nanotube and Graphene products in Turkey.
Nanokomp has recently started producing a new high-end prepreg line, doped with carbon nanotubes and graphene. Thus, the ideal properties of graphene sheets and carbon nanotubes will be transferred from the atomic scale to the meso and macroscopic level (Shown in Fig.3). In other words, the exceptional features of individual 1-D and 2-D nano reinforcements will be passed to 3-D bulk materials, which will maximize the impact upon composite world.
By adding only 0.1 wt. % of carbon nanotubes into prepregs, this new generation composites provide improved compressive and flexural strength (improved by more than 5 % for both), tensile strength (more than 15 %) and electrical conductivity.
Doped prepregs are sufficiently conductive for total electric discharge by adding 0.1 wt. % carbon nanotubes and even reach EMI shielding effectiveness with 0.5-1 wt. % additions. Forward looking design projects are in process to further improve those properties and together with improving fracture toughness, thermal dissipation and flame resistance. The improved properties of the end-product attained by the incorporation of these nanofillers will be appealing for the high-added value applications, such as aeronautics, automotive and aerospace, where stability in extreme conditions, low-weight, high flame resistance, good UV radiation shielding and mechanical properties are required.
