JEC COMPOSITES MAGAZINE - Issue #41 - June 2008 - 65

Fig. 3: Rheology curve for hot melt SPRINT® incorporating dwell for optimum flow and wetting While prepregs have always given the best mechanical-performanceto-weight ratio when moulded properly, they are heavily reliant on the skills of the laminator to ensure a high-quality component. If little care is taken during the lay-up process, then a poor quality part will be produced. To overcome this, time-consuming processes like vacuum debulking of individual plies have evolved to ensure low void contents. SPRINT® overcomes the ‘skill factor’ by being easy to layup (a ply of SPRINT® has the same conformability as a ply of dry fabric), and by eliminating the need to chase out air as in between sticky plies of prepreg. In fact, by sandwiching unidirectional prepregs in between layers of SPRINT®, it is possible to have the best of both worlds – the optimum mechanical performance of unidirectional prepregs and the air-breathing power of SPRINT® to produce a high-quality and highperformance laminate. The very nature of SPRINT ® also means laminators are effectively laying two plies of reinforcement into a mould with a single ply, so inherently halving labour costs. dwell is usually inserted into the cure cycle while the resin is mobile to allow plenty of time for the reinforcements to fully wet out before advancing to the final cure temperature. SPRINT® surface films – Reducing finishing costs It has been widely accepted by many commercial sectors, from aerospace and marine to wind energy, that the mechanical performance of continuous fibre-reinforced epoxy composites offers the best balance of properties to make a structure stronger and stiffer, yet lighter. However, surface finish and the associated costs of repairing and fairing imperfections have long contributed a hidden cost to composite structures. By using the SPRINT® ability to breathe air away from a mould surface, it is possible to formulate lightweight surface films and eliminate surface porosity entirely. In the automotive market, the surface finish issue dogged composites right from the early days. By using the SPRINT® surfacing technology, it is now possible to overcome the previously insurmountable issue of ‘print through’. This is where the weave of the underlying fabric is clearly and unacceptably visible through the paint and topcoat. This can be initiated by temperature and humidity, and relates to the difference in coefficient of thermal expansion (CTE) between the resin (high) and the fibres (low). It is possible to achieve a ‘Class A’ surface finish by formulating to reduce the CTE of the resin so it is closer to that of the reinforcing fabric in the SPRINT® layer. This world-leading technology has enabled SPRINT® to be selected for body components manufactured in-house at Gurit UK for the Aston Martin, as well as for the supply of surface films and structural SPRINT® CBS to the likes of MG Rover and TVR. Similar surfacing materials (lower cost for less demanding eyes) have been formulated to reduce finishing in the wind energy and marine markets. Shopfloor-friendly by chemical design There are many practical aspects to be addressed to make a product which can survive on the shopfloor. First and foremost is making sure that the resin does not wet out the reinforcement at room temperature on the roll. This resistance to ‘cold flow’ of the epoxy resin is critical to ensuring that the dry fibres in the reinforcement allow effective air transport out of the SPRINT® layer. It is possible to dial in the optimum rheology for SPRINT® by playing around with the chemistry of the epoxy resin at molecular level. In this way, extremely high levels of viscosity can be achieved at room temperature when the resin has a thick ‘toffee-like’ consistency, yet falls sharply in viscosity to a very mobile liquid similar to engine oil as the oven is switched on and the curing cycle commences. A No41 June 2008 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #41 - June 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #41 - June 2008