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The Advanced Composites Group (ACG) recently developed a range of products that are aimed specifically at vacuum bag processing of large structures for the marine, aerospace and wind energy markets. These products are based on the patented ZPREG “selective impregnation technology and VTM” 260, a Variable Temperature Moulding resin system offering a range of cure cycles from 16 hours at 65°C to 1 hour at 120°C depending on the size of the components, the temperature capability of the oven and the mould tool.
(Published on October-November 2005 – JEC Magazine #20)
BY ROBWILDER, MARINE SECTO R MANAGER, ACG
ZPREG is a new approach to composite material processing designed to produce a material that is quicker to lay-up, easier to use and produces higher quality laminates than other techniques such as prepreg lay-up, wet lay-up, resin infusion or resin transfer moulding. The material comprises two layers of d ry reinforcement, which are selectively impregnated with stripes of epoxy resin as shown in figures 1 and 2. Once the component is laid up, the material is consolidated under a vacuum bag and heated. This causes the resin viscosity to drop and the resin stripes to flow and impregnate the reinforcement as shown in figure 2.
A move towards prepregs
Traditionally, the boat building industry has used wet lay-up techniques with polyester, vinyl ester and epoxy resin systems. There has recently been a move towards prepregs for a number of reasons: Increasingly stringent health and safety legislation is making it difficult to use polyester wet lay-up resins, particularly for large structures due to styrene emissions.
The use of a pre- impregnated material gives far more accurate control over the resin- to- fibre ratio and hence more control over the final weight of the finished stru c t u re. With wet lay-up techniques, the maximum fibre volume fraction achievable is around 40%. The use of pre- impregnated materials allows a fibre volume fraction of up to 50% to be used, resulting in lighter, stiffer structures. For the manufacture of the large components typical in the boat building industry, techniques such as wet lay-up and vacuum infusion are difficult to accurately control.
The use of pre-impregnated material gives a lower-risk manufacturing route, eliminating the concerns that the use of large volumes of pre-catalysed room temperature curing resins create. Compared to conventional fully impregnated prepreg, the ZPREG material format has significant advantages:
The low tack nature of the material means that it has excellent handling and drape characteristics, more like a dry fabric than a prepreg. Laminating times compared to conventional prepregs can be reduced by up to 70%. Furthermore, there is limited contact with wet resin, giving a cleaner working environment.
The “in-plane” and “through-thickness” air paths eliminate the trapping of air that occurs with conventional prepregs, meaning that no vacuum debulks are necessary and that the cured laminate has exceptionally low voidage levels (
The air transport architecture of the material means that laminates with an excellent surface finish can be achieved without the need for autoclave consolidation pressures, gelcoats or epoxy primers.
ZPREG materials are available with a wide range of glass, carbon and Kevlar woven and stitched reinforcements. Fabric weights can be as low as 200gsm for lightweight detailed components and up to 600gsm for more substantial structural uses. One ply will have two fabric plies meaning that up to 1200gsm of material can be laminated in one pass.
Exceptionally low porosity
ZPREG materials can be combined with standard fully impregnated prepregs to aid air removal during cure. For high-performance structures such as racing yachts, masts and spars, the majority of the laminate lay-up would normally be unidirectional in order to achieve the maximum possible laminate stiffness. Such structures are normally manufactured using lightweight unidirectional prepregs with areal weights between 200 and 300gsm. For optimum perf o rmance, these prepregs are usually manufactured with low-viscosity “zero- bleed” epoxy resin systems such as ACG’s VTM266. The low viscosity of such a resin system results in exceptionally low porosity within the laminate. However, this low viscosity means that care has to be taken when vacuum bagging the laminate for final cure.
Conventionally, when vacuum bag curing a large prepreg structure such as a hull, a perforated or pin-pricked release film will be used between the uncured laminate and the breather felt as shown in figure 4. This ensures that there is a through-thickness air path from the laminate stack into the breather felt.
With low-viscosity prepreg systems manufactured with low resin contents for weight-critical structures, the use of a perforated release film can cause excessive amounts of resin to bleed out of the laminate stack into the breather felt, causing resin starvation and localised voidage within the laminate. The use of intermittent plies of ZPREG woven reinforcements or a ZPREG peel ply can overcome this problem.
As shown in figure 5, a “solid” (non-perforated) release film can be used in conjunction with some i m p regnated peel ply to provide a reliable air path across the laminate during cure. This technique eliminates the uncertainty of guaranteeing a reliable air path and high vacuum levels over large-area laminates.
Recently, structures such as Americas Cup Class hulls and decks have been successfully manufactured using this technique. Microscopy of sections taken from these laminates show exceptionally low porosity with voidage levels of less than 1% being measured.
The ZPREG concept has been further developed with a new resin system specifically formulated for the manufacture of large-area composite tooling. LTM317 is a third-generation development of ACG’s LTM10 and LTM200 series systems. Compared to other commercially available epoxy prepreg tooling systems, this innovative new resin system has the following advantages: - a long outlife of 21 days at room temperature allows very large tools to be laid up without the worry of the material curing during the laminating period. Traditional tooling prepregs have a limited room temperature outlife meaning that the entire section has to be laid up and ready for cure within five days. For large-area tools, this can be a significant problem with numerous laminators, long working hours and meticulous planning required. LTM317 can be easily laid up and prepared for a 65°C oven vacuum bag cure without the stress and logistics created by the use of a short-outlife material, - LTM317 can be initially cured at temperatures as low as 65°C. This allows the use of pattern materials manufactured from timber, Styrofoam, fibreglass and other low-cost materials, - with a suitable post-cure, the glass transition temperature (Tg) of the resin system will step ahead of the cure temperature. This allows large moulds to be removed from the pattern after a low-temperature cure and then fully post-cured to give a final Tg of 170°C, - a low-viscosity variant is available for the impregnation of heavyweight reinforcements. This allows the use of heavy bulk plies up to 1300gsm in areal weight. For the manufacture of large, simple geometry mould tools, the use of such heavy materials can significantly reduce lay-up times and labour costs, - with the use of ZPREG surface plies, tools can be manufactured with exceptionally low levels of surface pitting comparable to an autoclave cured laminate. This eliminates the need for a gelcoat, substantially increasing the tool life and simplifying the tool manufacture.
This technology will allow the manufacture of lighter, stiffer and more durable structures using cleaner, quicker and easier manufacturing techniques.
The ABN AMRO Volvo 70 female tool
The first of the new Volvo 70’s round-the-world racing yachts was launched in January. This new class of ocean race boats is designed to meet the toughest of conditions and withstand months at sea.
ABN AMRO 1, built using the VTM260 series prepreg system and the more traditional boat building approach of moulding directly off a male tool produced from plywood sheathed in glass, has been highly successful in initial sea trials.
For the second of the ABN AMRO Volvo 70s, a diff e rent approach was taken after builder Killian Bushe decided, in discussions with ACG’s marine team, that moulding the hull from a female carbon mould tool would offer great advantages to the yacht designer Juan Kouyoumdjian. The weight savings that could be achieved using a female mould were huge since less filling and fairing would be required. Thus, not only would the build team save weight, they would also save valuable build time – all important considerations for an offshore race boat.
The ZPREG‚ and LTF3‰18 tool lay-up suggested by the team at the ACG Technical Centre was both quick and easy. The surface area of the ABN AMRO 2’s hull is around 150 sq. metres and Killian Bushe’s team managed to lay-up and debulk the whole hull tool in just under 3 days.
A similar tool made using liquid infusion technology could realistically be expected to take twice as long to manufacture and not produce the same consistent level of laminate quality or surface finish.
Following a cure at 65°C for 16 hours, the tool was stable and ready for de-moulding and then post-cure. The surface finish of the completed tool was pit free and ready for the lay-up of the second Volvo 70, which is currently under construction using VTM260 prepreg.
The Volvo 70 mast tool
High-quality composite tooling is no longer the domain of the autoclave. ACG’s new generation vacuum-only processing tooling systems now offer rapid turn round, affordable, high-surface- quality composite tooling without having to re s o rt to high-pressure curing. Just what was needed when Lymington-based Formula Yacht Spars tasked with building masts for the Volvo Round-the-World- Race needed a 35-metre tool at short notice from a master not able to take either high temperature or autoclave pressure.
LTF318B, ACG’s ZPREG‚ partial impregnation surfacing technology combined with LTM‰317 prepregs, gave Formula Yacht Spars the balance of rapid lay-up, low temperature (65°C) and pressure (vacuum bag) initial cure and final service temperature of 120°C they needed.
The tool, laminated with support from the ACG technical department, required no refinishing yet it was trimmed, bolted up and post-cured and ready for action in a matter of days!
Flying International Moths
Without a doubt, the most talked about area of sailing so far this year has been the International Moth Class and the recent breakthrough in the foiling technology, which is now allowing these boats to fly! The recent world championship held in Australia was won by one of the new flying Moths built by Fastacraft of Perth, Australia that uses the VTM264 resin system from ACG.
The hulls, made from a simple carbon laminate over a PVC foam core, are cured between 80°C to 90°C under vacuum in a regular oven. Typical bare hull weights for these 3.3m dinghies is just 4.7kg, around 9.2kg finished with paint. A fully-rigged boat, ready to sail, is just 30kg! The VTM260 series system was chosen for its good tack and fast cure cycles.
The International Moth class is now seeing boats sailing around 20% faster than the previous non-foiling boats, now affectionately known as “sea-huggers”, with speeds hitting up to 25 knots.