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The use of composite materials in the automotive industry is not a recent occurrence, with low-volume “niche” vehicles taking advantage of composites’ high stiffness-toweight ratio for many years. In general, the benefits of replacing stamped metal with composite components are well understood and where possible, have been utilized.
(Published on August-September 2005 – JEC Magazine #19)
BY PETER CATE, AND SCOTT BURR, THE DOW CHEMICAL COMPANY
However, it has not been possible for mainstream vehicle manufacturers to adopt composite techniques to levels seen in other industries such as marine or aerospace. The fundamental reason for this is the vehicle build-volume requirements, which demand much faster cycle times than traditional composite solutions can provide. The slow cycle times have a significant cost implication which often cannot be overcome despite the other valuable attributes of the composite solution.
For certain applications, it has been possible to produce relatively rapid compression mouldings (for example with a polypropylene matrix) for high-volume applications. However, these are typically limited to only semi-structural applications, simply because the materials used are not capable of providing the appropriate levels of stiffness and durability required to replace metal in truly structural (load-bearing) applications.
Several studies have been carried out by major manufacturers, to investigate high-performance carbon-epoxy or glass-epoxy composite solutions, with promising results using the resin transfer moulding process. However, there are three core reasons why this approach cannot be realistically be implemented in production: - cycle time – the structural epoxy products require a certain time to penetrate and wet out the fibre reinforcements in the tool, as a function of the resin viscosity. This leads to long cycle times (for example more than 15 minutes for a large component) which is unacceptable for volume production, - performance – for an engineer to replace metal with composite, he has to be able to depend on that material’s ability to retain performance over the vehicle life, and not be susceptible to stress cracks or catastrophic failures. Structural epoxies, whilst providing adequate stiffness, are brittle by nature and cannot provide sufficient durability or impact resistance without a loss of stiffness, - environmental concerns – today the industry is faced with Endof- Life Vehicle (ELV) directives, detailing a specified weight of recyclable material within the vehicle. Thermosetting epoxy systems are by definition not economically recyclable and therefore will negatively affect the amount of recyclable material in the vehicle.
A unique raw material
Dow Automotive has spent a great deal of time understanding these important customer issues, and explored several material options within its broad plastics portfolio, before settling on CBT™ oligomers as a raw material, despite the fact that it is not manufactured internally. CBT™ oligomers are produced by Cyclics Corporation in Schwarzheide, Germany, and Dow has an exclusive agreement with Cyclics to supply the automotive market with this material.
CBT™ oligomers represent a truly unique raw material. With addition of catalyst and heat, it can be polymerized in the moulding tool to create pCBT composites of revolutionary performance. In this case, the low viscosity of the CBT™ melt permits the use of processing technologies reserved for high-flow thermosetting materials, such as low-pressure compression moulding, RTM, VARTM and others. These composite production processes are of great interest, given the tremendous focus on vehicle weight saving, driven by fuel efficiency targets. The weight of the average passenger vehicle has increased dramatically in the last five years, due to enhanced safety engineering and equipment as well as an increased level of electrical equipment.
High-performance structural composites clearly offer the opportunity to reduce vehicle weight by 30-50%. However, the slow cycle times associated with composite processes must be addressed if the industry demands for high-volume builds are to be met. It is calculated that in order to manufacture 30,000 parts per annum from a single tool, the cycle time must be in the region of 5 minutes in order to compete with a stamped steel solution (disregarding benefits of integration, shape complexity, light weight, etc.).
The CBT™ oligomer melt viscosity is demonstrably lower than epoxy, enabling faster injection and wet-out of the fibre reinforcements. The subsequent polymerization to pCBT is then rapid, leading to cure times lower than 5 minutes in an optimized process.
This ease and speed of CBT™ oligomer processing can therefore service the higher build volumes historically denied to structural composite solutions. However, it is not enough to process quickly: of course the performance of the parts must be sufficient for the applications considered, and durability is a key issue here. Polymerizing CBT™ oligomers in a moulding tool containing glass or carbon fibres produces thermoplastic composites with a remarkable stiffness-toughness balance in comparison with even high-performance thermoset systems such as vinyl ester epoxy resins.
Table 1: performance comparison with vinyl ester resin.
It is clearly seen that pCBT composites provide increased toughness, strength and resilience with the same stiffness, in addition to good temperature resistance. The low viscosity of pCBT also enables a high weight percent of fibre to be used, maximizing performance. If carbon fibre reinforcements are then used instead of glass fibres, stiffness is dramatically increased. For example, a 62wt% unidirectional pCBT carbon-fibre composite shows a tensile modulus of 115GPa with a flexural strength of 1,300MPa.
Table 2: 0-90 Stitch bonded carbon-fibre pCBT composite (1600g/m2 in both directions
This high performance, coupled with increased durability and fatigue resistance, offers a more predictable material for the engineer, enabling pCBT to be used in a range of semi-structural and structural automotive applications, from engine undertray to complete structural floor-pan for example. In addition, the material is suited to large composite components in bus, truck and rail applications where weight saving is sought but where thermoset systems do not provide an adequate property balance.
When environmental issues are taken into account, the thermoplastic nature of pCBT is of great benefit. Using composites to replace large quantities of recyclable steel in the vehicle structure will undoubtedly reduce the percentage of recyclable weight available. Therefore, it is critical that the composite itself is also recyclable, to recapture some of this percentage. In this, epoxy and other thermoset systems have been unable to offer an economically recyclable solution to meet End-of-Life Vehicle (ELV) directives.
Dow is developing several solutions to service the composite market. Relatively simple geometries can utilize powder “prepregs”, in which fibre reinforcements are pre-impregnated with a one-component system (unpolymerized CBT™ oligomers plus catalyst). The resultant prepreg is then compression moulded at very low pressure, to form the final shape. A key benefit of this process is the lack of volatile organic compound emissions during polymerization. The polymerization of the CBT™ oligomers is rapid, with cycle times of less than 5 minutes with Dow’s faster catalysts.
Collectively, the attributes of pCBT provide the moulder and carmaker with the opportunity to bring composites to the mainstream automotive industry, not only with a high stiffness, tough material, but one that is also an environmentally friendly thermoplastic.
In addressing the key customer needs in the composite applications targeted, Dow has succeeded with pCBT in creating the next generation of high-performance composites for the automotive industry.