THE ROLE OF RECYCLED CARBON FIBRES IN COST EFFECTIVE LIGHTWEIGHT STRUCTURES Frazer Barnes ELG Carbon Fibre Ltd Abstract Recent years have seen the development of commercial operations for the recovery of high grade carbon fibres from manufacturing and end-of-life wastes. Two challenges faced by this developing industry are the conversion of recovered fibres into usable product forms, and the acceptance of these products by the market. This paper describes the development and testing of recycled carbon fibre products that have the potential to enable cost effective. lightweight structures in transportation, the products being reinforced thermoplastics designed for injection moulding and nonwoven textiles designed for composites manufacturing. The technical performance of these materials is compared with current materials, and the economic and environmental benefits are highlighted. Finally, the challenges that have still have to addressed before the materials become widely accepted in the market are discussed. Background and Requirements The last decade has seen a significant increase in the use of carbon fibre, with this trend forecast to continue. The increase in the use of carbon forecast has highlighted that this is an industry where a high percentage of the raw material ends up as manufacturing waste as a result of the lack of a commercial recycling solution. It is estimated that more than 2, tonnes of carbon fibre manufacturing waste was generated in 215, and that less than 1% of this was recycled. Recognition of the need to deal with this material not as waste but as a valuable, secondary raw material has led a number of companies to develop methods of reclaiming carbon fibre from waste. Whilst all of these initiatives have had their challenges, today several companies are able to reclaim high quality carbon fibres from manufacturing waste. However, despite this capability existing, less than 1% of global carbon fibre waste was recycled in 215. Two of the main reasons for this are (1) : The limited availability of design and manufacturing data for recycled carbon fibre products. The need to develop new markets and applications for recycled carbon fibre products, due the challenges of replacing prime carbon fibre in current applications. Page 1
Whilst much of the carbon fibre industry s growth so far has been based on increased use in aerospace and wind energy, future growth is anticipated to come from the new and emerging markets, such as automotive body structures and components. Although there has been increasing use of carbon fibre in the automotive industry, in general it s use has been limited to sports or luxury vehicles which are better able to absorb its high cost. In order to bring the lightweighting benefits of carbon fibre to a broader range of automotive applications, it is necessary to reduce the cost of the material. One approach, which may prove successful in the longer term, is to develop and bring into production lower cost forms of carbon fibre. Another approach, which is available immediately, is to recycle the high volume of carbon fibre waste that is produced today into products that can be used in the automotive and other high volume industries. The process for reclaiming carbon fibre is well established. Extensive testing of fibres before and after the conversion process using Favimat single filament testing equipment shows a reduction of tensile strength of approximately 4% and a reduction of tensile modulus of less than 2% after the pyrolysis process (Figure 1). 5 3 45 4 25 Tensile Strength MPa 35 3 25 2 15 1 5 Tensile Modulus GPa 2 15 1 5 Impregnated Strand Tensile Strength Pre-furnace Single Filament Tensile Strength Post-furnace Single Filament Tensile Strength Impregnated Strand Tensile Modulus Pre-furnace Single Filament Tensile Modulus Post-furnace Single Filament Tensile Modulus Figure 1: Single Filament Tensile Testing of Fibres Before and After Pyrolysis The main challenge in using reclaimed carbon fibres is how to convert them to a form in which they can be reused. This is due to the difference between prime carbon fibre continuous, well aligned bundles comprised of many thousands of filaments and recycled carbon fibre individually filaments of varying length with very low bulk density and which, importantly, do not have the characteristics of textile fibres. The two areas with the greatest potential to use recycled carbon fibres are for reinforced thermoplastic compounds and compression moulded composites. This paper now discusses how recycled carbon fibre products have been developed for these applications. Page 2
Recycled Carbon Fibre Products for Reinforced Thermoplastic Compounds The global market for short fibre reinforced thermoplastics is 3.55 million tonnes (2). Approximately 54% of these materials are used in the transportation sector, with some of the higher volume applications being air inlet manifolds (61k tonnes) and engine and cam covers (126k tonnes). The most frequently used polymers in these applications are polyamides (PA66 and PA6). Other polymers of interest are polypropylene and polycarbonate. Today only 2, tonnes (.6%) of these products use carbon fibre as the reinforcement, with the remainder of the compounds being reinforced with glass fibre. However, that percentage is forecast to increase to.4%, or 16,5 tonnes, by 221, requiring more than 3, tonnes of carbon fibre. Interest in carbon fibre comes from the possibilities of using a reduced quantity of a lower density reinforcement to achieve the same properties as a glass fibre reinforced compound, or the same quantity of reinforcement to provide higher mechanical properties and allow the materials to be considered as replacements for metal castings. However, the applications of carbon fibre reinforced compounds are limited by the high cost of the fibre. Recycled carbon fibres, at typically 5%-6% of the cost of prime carbon fibre, offer the potential to obtain the same performance advantages of prime carbon fibre at much reduced cost. The barriers to using recycled carbon fibre include the means of introducing the fibre into the compound. Whilst heavily sized prime carbon fibre tows can be chopped and easily fed into compounding lines, handling of recycled carbon fibres presents more challenges because of its low bulk density and the fibrous nature of the product. In developing a product that addressed this, factors that had to be considered included: The product had to be free flowing and capable of consistent dosing into a compounding line. The product had to dust free. The product had to maintain sufficient fibre length to ensure adequate mechanical properties in the final compound. The product had to contain a sufficiently high proportion of carbon fibre to allow it to be used as a basis for engineered compounds. Three approaches to meeting these requirements were investigated: Diced fibreboards. Pellets made using a ring die of flat die approach, as used for animal feed and fuel pellets. Pellets made using an extrusion approach. The diced fibreboard option was quickly discarded, as this option had the highest capital and operating costs, and the product that was produced was still difficulty to accurately dose and was dusty. Attention was then focused on investigating pellets made using the ring die and extrusion methods. For the initial investigations, polypropylene was used as the polymer for the compound. Page 3
Figure 2: Pellets Made Using Ring Die and Extrusion Processes Problems encountered with the initial pellets made using the ring die process included lack of compaction and fibre shortening from a starting length of 4mm 8mm to around 12µm in the final pellet. The fibre length problem was solved by redesigning the dies. After several iterations of die designs were tested, it was possible to produce pellets with an average fibre length of 7µm. The lack of compaction in the pellets was resolved by increasing the amount of binder used to make the pellet, however this was found to have negative consequences in terms of mechanical properties of the final compound. The biggest challenge faced in the extrusion process was feeding the low bulk density, chopped carbon fibre into the extruder. This was resolved through the use of a system in which the chopped carbon fibres are first force-fed into the side feeder, and then into the extruder. The advantages provided by the extrusion process are lower operational costs, with separate processes for binder application and drying being eliminated, and the ability to use a polymer binder that is fully compatible with the final compound. The pellets are also completely dust-free, which is an important attribute for the compounding industry. The disadvantage of this approach is that the maximum amount of fibre that the pellet can contain is approximately 4%, as opposed to the 9% fibre content of the ring die pellets. However, this is not a limitation for most applications. The mechanical properties of pellets made using these two methods were compared with those of pure polymer, and also with compounds made using chopped prime carbon fibre. The results of this testing are shown in Figure 3. Page 4
Tensile Strength MPa 7 6 5 4 3 2 1 % 5% 1% 15% 2% 25% 3% 35% Reinforcement Content Ring Die Pellet Extruded Pellet Prime CF Tensile Modulus GPa 8 7 6 5 4 3 2 1 % 5% 1% 15% 2% 25% 3% 35% Reinforcement Content Ring Die Pellet Extruded Pellet Prime CF Figure 3: Mechanical Property Comparison of Polypropylene Compounds The conclusion from this first phase of the work was that a compound made using a recycled carbon fibre pellet produced using the extrusion method was capable of providing the same or improved mechanical properties as a compound based on chopped prime carbon fibre. The next phase of the work was to investigate polyamide compounds. This was done using PA66 as the base polymer. Compounds made using glass fibre, recycled carbon fibre and chopped prime carbon were all investigated. The results of this testing are shown in Figure 4. Flexural Strength MPa 45 4 35 3 25 2 15 1 5 % 5% 1% 15% 2% 25% 3% 35% Reinforcement Content Glass Recycled Carbon Prime CF Tensile Modulus GPa 3 25 2 15 1 5 % 5% 1% 15% 2% 25% 3% 35% Reinforcement Content Glass Recycled Carbon Prime CF Figure 4: Mechanical Property Comparison of Polyamide Compounds Page 5
It can be seen from Figure 4 that the compounds based on recycled carbon fibres had very similar mechanical properties to those based on chopped prime carbon fibre. Another important finding from this work was that compounds reinforced with 1% recycled carbon fibre had higher tensile modulus than fibres reinforced with 3% glass fibre. With stiffness being one of the main design drivers for the typical applications of these compounds, this offers the opportunity of a 23% weight reduction due to the lower density of the carbon fibre reinforced compound (121 kg/m 3 ) compared to the glass fibre reinforced compound (156 kg/m 3 ). The lower weight of a carbon fibre reinforced PA66 component means that the material cost increase between a lightweight carbon fibre part and glass fibre part is less than 4%. When considered in the context of total part cost, the 23% weight saving can be achieved at a cost increase in the finished part of less than 2%. Having demonstrated the technical and commercial feasibility of a recycled carbon fibre product for the compounding industry, this project has now moved to a phase where the mechanical and physical properties of a range of compounds will be fully characterized and an initial production line to make the pellets will be installed. Recycled Carbon Fibre Products for Composites The importance of weight reduction as a means of meeting increasingly stringent fuel efficiency and emission reduction targets is well recognized. A study by Oak Ridge National Laboratory presented an extreme lightweight body-in-white concept that resulted in a 67% weight reduction compared to a traditional pressed steel construction, that also increased bending and torsional stiffness (3). Important points about the design concept included: Of a total use of 72.5kg of carbon fibre, 54.8kg (75%) was in the form of discontinuous chopped fibres. The main manufacturing processes were compression moulding, either of laminates or SMC. A carbon fibre cost target of $11/kg was identified as being necessary to enable high volume applications in the automotive industry. Minimum performance targets of 172 MPa tensile strength, 172 GPa tensile modulus and 1% strain to failure were identified for the carbon fibres. Performance targets of 15MPa-215 MPa tensile strength and 3 GPa-37GPa tensile modulus were identified for the carbon fibre laminates. Assessing reclaimed carbon fibres against these requirements, the fibres are already in a discontinuous filament form, the cost of the fibres is below the targets that have been established and the mechanical properties of the fibres exceed the targets that have been established. The remaining challenge is again processing the reclaimed into usable product forms. The goal of this work was to develop two product forms: A carbon fibre nonwoven mat, which could be used in liquid compression moulding processes or as the reinforcement in a prepreg or SMC material. A hybrid carbon fibre/thermoplastic nonwoven mat which could be used in compression moulding processes. Page 6
Thermoset Composites Although carbon fibres generally lack important characteristics of textile fibres, the starting point for this project was to use conventional textile processing machinery to assess its suitability to manufacture carbon fibre products. This early work demonstrated that it was possible to produce carbon fibre nonwoven mats from reclaimed carbon fibres, however the production efficiency and mechanical properties of the laminates made from these materials meant that neither commercial nor technical targets could be achieved. Following on from this, attention turned to modifying the equipment and process with the goals of producing a more consistent product at a commercially acceptable production rate. After several iterations, machine configuration and operating parameters were established that delivered a consistent product from reclaimed carbon fibres. A feature of these materials is the very even fibre distribution, across the range of fibre areal weights from 9gsm to 4gsm. Figure 5: Recycled Carbon Fibre Nonwoven Test laminates were made using resin infusion, high pressure RTM, liquid compression moulding, prepreg compression moulding and SMC. Figure 6: Laminate Test Panel Page 7
Conclusions from these trials were: Using resin infusion, fibre volume fractions in the range of 16% to 18% were achievable. Using liquid compression moulding, prepreg compression moulding and SMC compression moulding, fibre volume fractions up to 35% could be achieved whilst still producing high quality (low void content) laminates. Above 4% fibre volume fraction, laminate quality appeared to suffer and this was reflected in lower tensile and compression strengths. The high bulk factor of the nonwoven mats combined with the low permeability (a consequence of the very even fibre distribution) limits the usability of these materials in high pressure RTM processes. Tensile strength and modulus of the samples was measured to allow comparisons to be made between different processing routes and to assess the effect of fibre volume fraction. Some of these results are presented in Figure 7. Tensile Strength MPa 5. 45. 4. 35. 3. 25. 2. 15. 1. 5.. 2.% 22.% 24.% 26.% 28.% 3.% 32.% 34.% Fibre Volume Fraction Tensile Modulus GPa 4 35 3 25 2 15 1 5 2.% 22.% 24.% 26.% 28.% 3.% 32.% 34.% Fibre Volume Fraction Figure 7: Mechanical Properties of Recycled Carbon Fibre Nonwoven Laminates It can be seen from Figure 7 that in all cases the tensile strength targets were exceeded, whilst the lower tensile modulus target was achievable using liquid compression moulding, prepreg compression moulding and SMC compression moulding at fibre volume fractions above 27%. In terms of consistency of mechanical properties, the coefficient of variation of laminates made using compression moulded prepreg and liquid compression moulding was around 4% for both tensile strength and tensile modulus. For laminates made using high pressure RTM, this increases to approximately 8% for tensile strength and 6% for tensile modulus this higher variability being attributed to fibre wash during the RTM process. The next phase of this work will focus on more detailed materials characterisation of the laminate physical and mechanical properties, and the development of design data, for laminate made using compression moulded prepreg and liquid compression moulding. Page 8
Thermoplastic Composites Thermoplastic composites offer potential advantages in terms of high rate manufacturing, ease of material handling and damage tolerance. One of several forms of thermoplastic composite material is the organosheet, a nonwoven mat comprising a reinforcing fibre (usually glass fibre but sometimes a natural fibre or carbon fibre), which can be heated and press moulded to form the final product. An example of the use of a carbon fibre organosheet is the CAMISMA project (4), where a carbon fibre organosheet was used to make a seat back. The goal of this project was to assess the technical feasibility of producing an organosheet based on reclaimed carbon fibres. Using the same equipment and process developed to produce carbon fibre nonwovens, it was possible to manufacture nonwovens based on blends of reclaimed carbon fibre with polypropylene, polyamide and PPS fibres. Mechanical properties from a reclaimed carbon fibre / polypropylene blend at a nominal 3% fibre weight fraction are shown in Table I below. Table I: Mechanical Properties of Recycled Carbon Fibre Organosheet Test Direction Tensile Strength Tensile Modulus Mean CoV Mean CoV Roll Direction 159 MPa 1.% 13.5 GPa 4.1% Cross Web Direction 24 MPa 1.7% 15.6 GPa 1.% Having established the feasibility of producing organosheets from reclaimed carbon fibre, work is now ongoing to investigate the range of fibre volume fractions that can be achieved, and the resultant mechanical properties, with a variety of thermoplastic fibres including polypropylene, polyamides and PPS. Environmental Benefits The focus of the work presented so far has been to develop ways of using carbon fibre manufacturing waste, which would normally be sent to landfill, as the basis for cost effective, high performance materials which can be used for lightweighting in the automotive sector. Making lightweighting more cost effective will enable car manufacturers to pursue more aggressive lightweighting strategies, thereby delivering greater environmental benefits. The potential to use carbon fibre in the automotive industry far exceeds current supply capacity. Reference (4) estimates the potential use of carbon fibre in the automotive industry to be in excess of 45, tonnes per year (or 6kg of carbon fibre per vehicle) if cost challenges can be addressed. Whilst it would be unrealistic to base estimates of environmental benefits on these volumes, what can be said is that in the medium term, a realistic expectation might be for 75%, or 15, tonnes, of carbon fibre manufacturing waste to be reused in the automotive industry each year. In environmental terms, this means: 45, tonnes of CO 2 emissions avoided each year, through the use of recycled carbon fibre instead of prime carbon fibre (5). 321, tonnes of CO 2 emissions avoided each year, through the use of carbon fibre instead of aluminium (5) as the means to achieve weight saving. 91, tonnes of CO 2 emissions avoided each year, by weight reduction compared to steel (6). 39 million litres (9.8 million gallons) less gasoline use per year, by weight reduction compared to steel (6). Page 9
This illustrates the environmental benefits that can be gained by making use carbon fibre cost effective through the use of recycled materials. Summary and Next Steps A first generation of recycled carbon fibre products that can be used to produce carbon fibre reinforced thermoplastic compounds and composite structures have been developed. Initial testing has shown that these materials offer substantially the same weight saving potential as prime carbon fibre, at a significantly lower cost. The affordability of these materials offers OEM the possibility of pursuing more aggressive weight saving strategies without significant financial penalties. Apart from the well-understood environmental benefits of weight saving in terms of reduced fuel consumption and lower emissions, the use of recycled carbon fibre materials reduced greenhouse gas emissions during vehicle manufacturing, as a result of the lower embodied energy in the recycled materials. The next step is to introduce these materials into series production. In support of this, more detailed design data and processing knowledge are now being developed through a number of demonstrator projects and additional capacity for both fibre reclaiming and conversion is being planned to support the anticipated increase in demand for these products. Acknowledgements The development of recycled carbon fibre pellets was partially funded by Finance Birmingham under an AMSCI WMLCR Grant. ELG Carbon Fibre would like to acknowledge the contribution of the University of Warwick with regard to testing short fibre reinforced thermoplastic compounds and performing life cycle analyses in this project. ELG Carbon Fibre would also like to recognise the contribution of Albis UK in manufacturing and testing some of the short fibre reinforced thermoplastic compounds referenced in this paper. Bibliography 1. Recyclable Composites Must Still Be Reused. Composites World Feb 216. 2. Growth Opportunities in the Global Short Fibre Reinforced Thermoplastic Composites Market 216-221: Trends, Forecast and Opportunity Analysis. Lucintel, April 216. 3. Low Cost Carbon Fibre Overview. C. David Warren, Oak Ridge National Laboratory, June 21. 4. http://www.johnsoncontrols.com/insights/215/automotive/feature/camisma-project 5. Calculation of Greenhouse Gas Emissions for Selected Material Flows of ELG Haniel GmbH for 214. Fraunhofer UMSICHT November 215. 6. The Potential Contribution of Light-Weighting to Reduce Transport Energy Consumption, Helms and Lambrecht, Institute for Energy and Environmental Research (IFEU) Heidelberg. Int J LCA 26:7 Page 1