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New prepreg systems with low-temperature, low-pressure processing and flexible cure temperatures are offering the marine industry high performance and surface quality via low-cost processing routes.
(Published on September 2007 – JEC Magazine #35)
STEPHEN MILNER, TECHNICAL AUTHOR ADVANCED COMPOSITES GROUP LTD.
What exactly is a prepreg?
Basically, a prepreg consists of a fibre reinforcement, i.e. carbon, glass, etc., in unidirectional (aligned) or fabric (woven or multiaxial) form, impregnated at a predetermined level with a resin matrix. This stringent control of resin content and dispersion eliminates the risk of fibre content variation which ultimately controls laminate performance, while the development of resin-rich areas can have an adverse effect on material performance. These types of problems can plague processes such as wet lay-up and vacuum infusion, perhaps even detracting from final material performance. While a number of material “enhancements” have recently been developed to aid processing, these problems are often detrimental to final component performance in terms of the achievable fibre fraction and component weight.
An additional benefit to using prepregs is that they remove the vagaries of resin chemistry; all formulation and mixing is carried out under controlled conditions, with no further handling of potentially harmful chemicals. Consequently, all health and safety issues are minimized. Similarly, the resin type, content and dispersion within the reinforcement can be fine-tuned to suit specific process requirements. It is even possible to introduce air paths within the reinforcement architecture, thereby allowing threedimensional air release during the early stages of cure, as seen in ACG’s ZPREG® formats. Such a process can reduce the risk of void establishment in the final laminate and even promote higher quality surface finishes. The inherent prepreg “tack” can be adjusted to meet specific requirements, allowing accurate positioning of the reinforcement on a mould and promoting a high level of fibre orientation control with no risk of fibre movement or wash-out during processing.
Bespoke composite solutions
ACG wanted to evolve from simply supplying “stock” material and holding large stocks of reinforcement fibre to providing bespoke product formats in short lead times, a philosophy born out of their longstanding relationship with Formula One. The company also believes in supporting and demonstrating its own technology. Recent programmes have seen a cross-over from Formula One into America’s Cup spars through to rapid tooling systems from aerospace being used in both Volvo 70 and America’s Cup builds.
Once completed, a prepreg laminate requires a combination of pressure and heat to consolidate the layers and initiate the resin curing reaction, which usually means resorting to a hightemperature autoclave – basically a large pressurized oven.
Typical curing conditions require 120°C and a pressure of 3 to 6 atmospheres (bar).
While an autoclave remained the ultimate route to producing high-performance components, it was seen as a barrier to the wider adoption and application of composite materials in other industries.
Innovative resin formulation has always been at the heart of ACG technology; a modern Formula One race car could feature up to eight different resins within its structure.
ACG has always manufactured “standard” 120°C curing prepregs, but an early advancement in this area led to a unique technology known as “step ahead” curing. This technology is centred on the company’s Low Temperature Moulding (LTM®) processing resins. These resins are formulated to cure at low temperature and can be transformed by further heat treatment into highly robust structures. These resins develop a higher level of performance in terms of mechanical and thermal characteristics than would be expected from a particular curing condition. This enhanced stability allows initial curing to take place at 50-60°C, thereby utilising relatively low-cost tooling. The cure can then continue to a much higher level with stability still running ahead of the cure temperature, but with the part now out of the mould and with no risk of distortion. This concept was first exploited in the LTM® tooling systems. The resins developed for this purpose exhibited resistance to extremely high temperatures, but their inherent brittleness made them totally unsuitable for structural applications. ACG’s determination to move forward into other arenas brought about further developments to this pioneering system, introducing toughness and allowing low-pressure processing, all essential characteristics that have made prepreg technology a reality for the marine industry.
That’s not to say these materials were perfect; material is stored frozen to maximize life, but the reactivity of a particular prepreg system and its minimum curing temperature are inextricably linked. Therefore, low-temperature curing means a short working life, which in turn limits the project size or dictates the speed at which you must work. This was less than ideal, but some early projects did show that these materials were attractive to the marine industry. This was reflected in a number of boats built at that time, including an 18ft Skiff, an Ultimate 30 and an Open 50. Race boats are not the only specialist area producing extreme one-offs. LTM® systems have also found use on a number of high-profile aerospace projects, including Scaled Composites’ Spaceship One and Global Flyer.
Interest in high-performance composites was growing and there was an obvious need for more user-friendly, low-temperature, lowpressure moulding solutions. That’s when ACG’s Variable Temperature Moulding (VTM®) system came onto the scene. The basic chemistry is derived from the LTM® system, offering optimised low-temperature, low-pressure processing, flexible cure temperatures between a modest 65°C and 120°C with “step ahead” cure and a working life of one month. The VTM® does not need an autoclave and resin rheology is controlled to optimise fibre wet-out, but not at the expense of excessive resin bleed. The myth still persists that resin must be bled from a component to maximise properties. ACG has always advocated “net processing” – what goes in comes out, resin content and material properties remain predetermined and controlled. Why buy resin just to throw it away?
The VTM® range has now grown into a family of structural, tooling, surfacing and gelcoat systems, providing the marine industry with high performance and surface quality via low-cost processing routes. In 2001, the America’s Cup was rolling into action, but this time with new players and new thinking. Having turned to Formula One for inspiration, Alinghi, the European challenger, was there to win, not to run with the rest of the pack. The rest is history. SUI 64 became the first major success for ACG’s VTM® system. Throughout the build of the SUI 64 and her sister SUI 75, ACG chemists and engineers worked alongside the build team and EPFL, a Swiss research group. Together they developed materials and process methodologies while cementing a close working relationship with the personnel in the Decision yard, where developments continue to be made to this day.
Despite VTM® system now being firmly established as the performance system of choice in the marine industry, ACG continued to explore ways in which prepregs can be made more attractive, particularly to series builders.
Composite materials are not cheap, and the material/labour cost balance can be even more critical in the production environment, so the viewpoint of “less is more” may be justified. Developments such as the ZPREG® rapid laminating and surface ply systems have been readily embraced by the marine industry, as has an epoxy prepreg compatible gelcoat, developed in association with Scott Bader Co. Ltd. Prepreg components are also making significant inroads into the production boat market. As race boat designers strive to meet increasingly tighter per-formance envelopes, focus has been concentrated on the tooling techniques adopted for one-off race boat builds.
Traditionally, boats are built over a male plug and the final shape achieved with a combination of a filler and a great deal of hard work, not to mention a price to be paid in labour time and additional weight. Continually striving to advance technology and techniques, ACG took yet another giant step forward and pioneered a process for the rapid manufacture of prepreg tooling, which combines its ZPREG® surfacing and rapid lay-up technologies.