Thursday, July 18, 2013

Prepregs boost marine quality

Using high performance fibres that are already impregnated with resin is, say the proponents of prepregs, a sure way of building consistent quality into fibre reinforced plastic (FRP) products. Aircraft builders became convinced some time ago, but constructors of marine craft have followed suit as customers for premium yachts and some commercial craft seek top performance with structures that are exceptionally light, stiff and strong.

Prepregs have become a primary means to achieve this winning combination. These materials avoid the nagging inconsistencies associated with traditional FRP processing, especially the variable quality achieved by different laminators working on different days in different conditions with non-consistent material mixes. Bulk material manufacture under controlled factory conditions ensures that prepregs provide high and consistent fibre-to-resin ratios, along with even fibre wet-out during cure. For their part, fabricators are happy to abandon messy wet lay-up and spray-up operations together with their associated volatile emissions. They can worry less about resin viscosity since this is controlled as a function of the cure, while concerns over precise mixing of catalyst, accelerator and resin are avoided. Less resin is wasted and laminators operate in a cleaner and healthier working environment. Multilayer materials simplify the lay-up process because some of the lay-up has already been done and is inherent in the material.

Although carbon epoxy composite has become a marine industry mainstay in recent years, glass and aramid prepregs are also available, with polyester, vinyl ester or epoxy resins. In the past, prepregs had to be cured under pressure at high temperature – typically 175°C for aerospace structures – and this held back any penetration into the marine sector, where most constructors have neither the desire nor the finance to acquire and operate large autoclaves. However, the advent of materials that cure at lower temperatures and with vacuum bag consolidation, with only minor sacrifice of performance and quality in the cured product, made prepregging viable for high-end marine craft constructors. Low temperature prepregs now available can be cured at atmospheric pressure (vacuum bagging) at temperatures as low as 65°C, avoiding the need for autoclaves and reducing energy needed during the cure cycle. Resin chemistry manipulation has made this possible. Prepreg suppliers use resins from various manufacturers, though some have developed their own bespoke formulations to meet specific customer needs.

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