Will Dalrymple explores some of the complexities surrounding GRP and the varying methods of joining different substrates
Glass-reinforced plastic is one of the great engineering collaborations in which the end result is greater than the sum of its parts. Filaments of glass spun and drawn by glassmakers are light and strong in tension. And although they tend to splay out in compression, that behaviour ceases when immersed in a thermosetting resin. And for their part, resins cohere well thanks to molecular cross-linking. But on their own, they cannot provide any particular strength, although they do provide structural shear resistance, particularly in between the layers of woven glass fibre. Combining the two offers strength from the fibre plus flexibility in form due to the resin’s plastic nature.
GRP starts as either a prepreg, in which resin comes on a roll with fibres, or the two parts are added together during production (pictured above). To turn a GRP matrix into a part, it must be inserted in a mould tool whose interior surface has been cut into a reverse of the final part shape. The GRP is held there, under compression and at the right temperature, until the curing process has completed.
The two most common types of resin in GRP are polyester and epoxy. Both are two-part systems. Polyester offers two significant advantages to manufacturers: low price and room-temperature curing, thanks to the fact that the chemical reaction generates heat.
By contrast, epoxy systems must be cured under heat at temperatures from 60-80°C for low-temperature systems, and typically 120°C up to 180°C, according to Chris Knight, engineering capability lead, materials and processes, at the National Composites Centre (0117 370 7600). Epoxy might cost an order of magnitude more than polyester, he points out, but then its mechanical performance is superior. In addition, the temperature at which the cured resin starts to soften, its glass transition temperature, is higher than polyester, at 100-180°C, compared to up to about 90°C for polyester. That means that epoxy-based GRP can provide a higher in-use temperature.
Whichever resin is used, the resulting material won’t rot and it won’t rust, two reasons why GRP is so often used in marine applications such as boat hulls. Composites are very resistant to weather and the elements – too good, perhaps, in that they will persist for hundreds of years, long after their users are gone. This is becoming a huge problem for the marine industry, as boats now beyond economic repair are being dumped at ports and harbours to avoid the costs involved of disposing of them responsibly.
As of now, that means landfill, as there are no commercial recycling systems for GRP: but the NCC is looking into the matter. As Knight puts it, “This is something of our own making, this problem, so we need to solve it.” It is investigating ways of reusing parts, recycling them into something of high value, rather than consuming energy to grind them up into feedstock.
Other research looks at pyrolysis – burning in the absence of oxygen – which consumes the resin and leaves the fibre behind, though that is also energy-intensive. And other methods under study involve liquid-based enzymatic processes decompose chunks of GRP boiled in a pressure vessel into fibre and resin.
Today, carbon fibre is often used instead of glass as the material backbone, particularly in motorsport and high-end applications, although its price remains an obstacle for many mass-market applications. One significant example where it is not used is in the manufacture of skis, points out Knight. And that is because glass is fundamentally more bendy than carbon: its Young’s Modulus is much less than that of carbon, 70-85GPa versus 200-450GPa, depending on grade. A bendy ski helps the person riding on them to negotiate bumps and undulations on snowy surfaces.
But in fact, creating a GRP to meet any strength or stiffness requirement is a complicated business, because it depends on the orientation of the reinforcing fibres. “You always get the best stiffness when you align the fibres in the force/stress direction. That’s how you use composites to tune the stiffness of your parts,” observes Knight.
This technique is not possible for cast or forged metals, because they are isotropic, and exhibit the same performance in every direction. Tuning metal structures to maximise performance per unit weight involves sculpting pockets or webs by cutting away material in CNC machining. (Or through using additive manufacture to produce topologically-optimised structures, although it remains relatively immature and costly.)
As a result, there is an engineering design risk with GRP. Knight adds: “If you don’t understand the benefits of composites or how to use them, your parts will use more material than they need to. That means they will be heavier and cost more and take more time than necessary to manufacture.” So, with some examination, there can be an opportunity to further reduce the weight of what is already a lightweight material.
Knight recommends that manufacturers approach their materials suppliers for help and advice – or, indeed, the NCC, whose mandate is to help industry through collaboration and trials. He also suggests education to develop expertise. NCC, among others, offers professional training, ranging from one-day introductions for managers to in-depth five-day courses on repairing damaged composite parts.
For that reason, he predicts that further development of composites like GRP is less likely to come from mechanical performance improvements than through greater application knowledge and expertise.
There are about half a dozen ways to join two composite parts mechanically, points out Lawrence Cook, development engineer at Bighead fasteners (01202 574601; so named because on the rear side of its range of collars and studs is a perforated metal plate used to embed it). The new white paper (see www.is.gd/zenite) published by the company’s corporate parent Bossard lists threaded inserts such as Bighead fasteners, multi-material welding, rivets, rivet nuts, bolts and nuts, or direct screwing using a thermoplastic screw (such as its Delta P2 product).
And, unlike carbon fibre reinforced plastic, GRP affords fabricators the potential to implant mechanical fasteners by manually moulding layers of matting over them. By comparison, carbon fibre is more difficult and expensive, says Paul Nelson, sales and marketing director at Bondloc Adhesives (01299 269269).
It is also possible to insert Bighead fasteners automatically into moulds, by holding the fastener in a cassette in a mould tool, according to Nick Waller, applications engineer at Bighead. Still, it remains unusual to embed fasteners in high-volume GRP parts production such as panels, he says. There are several reasons for this. First, the tooling might not allow fasteners. Another issue is the draw angle: when removing panels from tooling, if the fastener sticks out, it might prevent the part from being removed from the mould. Third, some panels aren’t thick enough to embed a fastener. Fourth, panels with embedded fasteners are more difficult to store and transport, as the projecting studs could damage other panels.
Another reason why fasteners might not be integrated, according to Nelson, is that the parts are made in multiple stages in an industrial supply chain. The assembler that does the fastening buys in GRP parts manufactured by another supplier, and the communication links between the two aren’t always sufficiently strong to collaborate.
For these reasons and others, GRP is one of adhesive supplier Bondloc’s fastest-growing markets for structural adhesives, particularly MMAs and epoxies, he admits. This is because many GRP bonding applications involve bonding dissimilar materials: GRP to stainless steel, aluminium, thermoplastics, and the supplier can tailor the choice of adhesive to meet the performance characteristics required. He states: “The advantage of an adhesive is that you can have a different one to deal with the properties of each substrate – it could be temperature, low modulus, high modulus, impact resistance or chemical resistance.”
The curing properties of MMA (methyl methacrylate adhesive) make it particularly useful for GRP fabrication. Due to the adhesive’s ambient cure, the time between the product’s working time and its fixture time is relatively fast, allowing parts to be bonded and moved on in the production process. Epoxy and PU systems mostly require a longer cure period to obtain their handling strength.
Another key parameter in choosing an adhesive is the rigidity of the joint intended. If the joint is too flexible, it can stress other parts of the system; if it’s too rigid, the part can break at the joint when under load. Adhesives mixing in 1:1 ratios tend to be the most rigid; those at 10:1 tend to be more flexible. MMAs come in both of those ratios; epoxies also come in other ratios, including 2:1 and 4:1.
Adhesives such as these are sold in hand-held cartridges for manual or pneumatic application, and in drums for high-volume applications. They are more likely to require dispensing machinery, adds Nelson: “We supply a drum, and adhesive is pressed out of that using a pneumatic delivery system, to apply a controlled application of that adhesive in some volume. If you’re joining a [boat] deck to a hull, you couldn’t do that with a cartridge – you need kg/min delivery in a confined space, and you need a bulk machine with a geared system [such as 2KM’s 20-litre capacity PolyMix machine, pictured right]. The bond line thickness and viscosity are so high, up to 1m centipoids, that there’s no way you could do it manually or pneumatically.”
In other words, the viscosity of the adhesive will impose its own requirements on the manufacturing operation. Nelson explains: “Last week I was working on a product that involved bonding three materials: ABS, aluminium and GRP. The adhesive choice was straightforward; it was about how they wanted to apply it and if they had enough time to put the parts together that was key.”
There again, the way that the final bonding process evolved affected the fixturing times required. “At the beginning, they were doing the job manually, and didn’t have a good understanding of standard operating procedures. Once they started to work out the best way to do it, [making] a product that needed 20 minutes of working time, they would be able to get away with five minutes to bring parts together, because they would get better with practice. At the beginning they wanted 10 units; hopefully they will produce hundreds. Prototyping is one thing, mass production another,” Nelson concludes.
Another cutting-edge project at NCC is the collaborative Wing of Tomorrow project, sponsored by the Aerospace Technology Institute, with Airbus, GKN Aerospace and Spirit AeroSystems as partners, that aims to create a completely composite wing skin at rates that exceed current manufacturing abilities using metallic options. The project aims to assemble all the structural elements of a wing, and then infuse a dry fibre with an epoxy resin system all in one shot. In doing so, the NCC’s Knight adds: “There is a huge benefit, because companies spend a huge amount of time and money assembling components. It also means that because the parts are co-infused and co-cured, the mechanical interface between them is much stronger than if they had been bonded after cure, or fixed mechanically by drilling holes and inserting fasteners or rivets. By co-curing, they achieve performance about as good as they can get in terms of mechanical performance.”