JEC COMPOSITES MAGAZINE - Issue #51 - September 2009 - 31

Prepreg How is it made? By definition, the dry reinforcing fibres or fabrics must become fully impregnated with the resin to become a prepreg. To achieve this, the resin has to be first cast into films of predetermined thickness and areal weight by heating the catalysed resin and coating it onto release coated paper. These films are then applied to either side of the reinforcement. By careful application of heat to lower the viscosity and make the resin more fluid, as well as pressure to force the now-liquid matrix into the fibres, the materials can be combined into a single prepreg product. laminating resin, whilst still being typically 25% lighter. Marine prepregs – the formulating challenge The type of prepreg used to make most structural aerospace components is based on formulated semi-solid epoxy resins. Unlike wet laminating and infusion resins, these materials do not have to include diluents in their chemistry and can be formulated without compromise to provide an excellent balance of structural mechanical properties, combined with high levels of impact resistance. The challenge for chemists such as those at SP the marine , business of Gurit, is to achieve similar levels of mechanical performance in aerospace prepregs whilst lowering the temperatures required to achieve such performance. They also need to manage this without compromising the handling and working time of the product. The earliest marine prepregs, such as SP’s Ampreg 75, met some of these requirements, e.g. a lower curing temperature of 75-80°C, though they were compromised in other ways. The consistency of prepregs made with Ampreg 75 was halfway between that of lightly tacky, non-transferring resins used in aerospace sector and that of a typical low-viscosity wet laminating resin. Shelf life was also limited to only three days at room temperature, so laminators frequently had to work through the night to ensure structures were laid up and vacuum bagged in time. Over 15 years ago, SP’s chemists overcame this problem with the development of SE84. This system combined SP’s knowledge of low-temperature curing with the same method of manufacture and high-performance resin constituents of aerospace prepregs. Adapting aerospace prepregs for marine structures Temperature and pressure are the two main issues that need to be addressed when considering the transition from an aerospace to a marine environment. Aerospace prepregs were historically split into two categories: 120°C and 175°C curing, these denoting the temperatures they had to reach for the epoxy system to react to its full extent. In the marine world, these temperatures are often difficult to achieve without using overly expensive heating systems and tooling materials. Aircraft CFRP components also typically use high consolidation pressures, 3 bar (45 psi) for honeycomb sandwich structures and 7 bar (100 psi) for monolithic laminates. These high consolidation pressures are generated by expensive pressure vessels called autoclaves. This ensures that the level of air which may have been trapped during the laying down of the individual prepreg plies is minimised. Any voids entrapped in the final cured structure will only serve to weaken the laminate and act as fatigue initiation sites. When working in the marine environment however, the sheer size and frequently one-off nature of a top race boat such as an America’s Cup yacht makes it nigh on impossible to contemplate building an autoclave capable of manufacturing a race boat. This is the reason why simpler compaction techniques have been developed, such as vacuum bag consolidation. Optimising structural design One of the key materials used in both aerospace and today’s marine prepregs is unidirectional carbon fibre prepreg, or ‘UD tape’. This is the purest form of any prepreg product, where the fibres are pulled off tensioned creel racks and held perfectly in column solely by the semi-solid resin matrix. Because there is no weaving involved, there is no crimp or ‘waviness’ in the carbon. Fig. 3: UD carbon compressive strength – Resin-dominated property. When combined with a well-formulated epoxy matrix, it is possible to achieve optimum translation of the carbon fibre properties and resin dominated properties, such as compressive strength which can be more than double those of a wet There was finally a material which combined the clean, safe handling of 120°C curing aerospace prepregs with the same No51 September 2009 / jec composites magazine 31

JEC COMPOSITES MAGAZINE - Issue #51 - September 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #51 - September 2009