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String and glue: A closer look at resin

Neil Webb
31 May 2016

Resin is the unsung hero that holds your carbon frame together, and it’s just as crucial to performance.

Ask most roadies what their bike frame is made from and the answer will likely be ‘carbon’. Ask anyone who’s involved in making bicycle frames (or indeed other products made from this woven wonder material) and you’ll get a more complex answer. 

‘In the bike industry we usually hear talk of carbon but really that’s too simplistic – a generalisation,’ says Thomas Leschik, head of engineering at German wheel manufacturer Lightweight. ‘It’s actually a matrix of carbon fibres and epoxy resin. The more accurate term is CFRP – carbon fibre reinforced plastic.’ 

So our highly coveted steeds are little more than strengthened plastic bikes. It’s a simple abbreviation, and goes a long way to explain the importance of resins – which are the plastic (or polymer) part of CFRP. Essentially the resin gives the composite material its rigidity. As Phil Dempsey of Aprire, a company specialising in carbon fibre bikes, puts it, ‘Carbon fibre is purely a fabric. Alone it’s just a piece of cloth.’

When it comes to product descriptions and the accompanying marketing spiel, the carbon fibre brand or type (eg Toray, T800, 65HM1K, ultra high modulus) is routinely trumpeted as the fundamental feature of the finished product’s characteristics. It’s as if there’s nothing else at play, but in fact the fibres make up just over half of the frame material. The rest is the epoxy resin, which clearly must play an important role in how a modern bike performs. Why, then, does the marketing blurb rarely mention it?

The ABC of CFRP

CFRP Carbon Fibre Reinforced Plastic (or polymer). The composite material referred 
to as carbon or carbon fibre.
Cure The process of applying heat and often pressure to a CFRP structure to ‘set’ the 
resin and afford rigidity to the finished piece.
Fibre Strands of carbon that are woven or knitted together to create the reinforcing element of a CFRP structure. Often called ‘filaments’.
Mould The physical component in and around which carbon fibre sheets are laid to create the frame.
Ply book Essentially a book of glorified sewing patterns. These detail how each individual piece of carbon fibre is cut and assembled, and they are the closest-guarded secrets.
Pre-preg Sheets of carbon fibre threads impregnated with uncured resin.
Resin A liquid polymer used to bond fibres together within a CFRP structure.

Insider knowledge

To understand the role resin has in a finished carbon fibre bike, we need to understand the manufacturing process, and how the resin is incorporated into it. 

In essence there are two types of carbon fibre construction: wet and dry. For wet manufacture, a company purchases carbon fibre cloth already impregnated with resin, known as pre-preg. These tacky sheets are laid up into or around a mould and then cured using heat and pressure to instill rigidity. The dry resin infusion manufacturing technique can take two different forms. The first is similar to the way pre-preg manufacture takes place, with cut shapes of dry cloth laid over a mould, with the resin added as part of the curing process. The second technique, used by companies such as Time and BMC (with its Impec bikes), involves stretching a continuous tubular sock-like structure over a mould in a single length. From here, the resin is added under pressure to the already formed shapes. 

Giant is the only brand that manufactures all of its own carbon pre-preg products from ‘spool to finish’ – that is to say it buys its carbon fibre as a thread on large spools, adds its own resin, and goes on to manufacture its frames, bars, stems and accessories. Giant, then, seems like a good company to ask about the importance of resins. 

Its UK product and training manager, David Ward, says, ‘Our carbon fibre filament is delivered direct from Toray [the world’s largest carbon fibre producer] to the spool room. From there it’s threaded to the looms and woven into huge carbon cloth sheets. It’s after weaving that the resin is added. The resin sits in a trough above the roller assembly and is passed onto the moving cloth, applied to the filaments via rollers.’ The process is a simple one, and the technique used by Giant is pretty much identical to that used by all manufacturers of pre-preg carbon fibre. But while it may be simple in its mechanics, the accuracy, repeatability and control are vital to the finished product’s integrity. 

‘The resin has to flow between and coat every single filament perfectly,’ says Ward. ‘Good resin distribution is vital to getting good pre-preg out of the end of a production line.’ Dempsey at Aprire adds, ‘It’s so important that the resin goes through the layers. If you get the resin wrong, you’ve got a cracked frame. It’s really critical.’

In the thick of it

‘Because resin makes up 40% of a Giant frame after curing, the resin is a very important part,’ says Ward. ‘Once it’s thermoset [cured], it’s the resin that gives the structure rigidity.’ In addition to basic structural properties, resin plays another vital role. Dempsey says, ‘You’ve got to transfer stresses from one part to another. It’s resins that allow the transfer of loads between the layers of fibres.’

Different resins will affect the performance of the final product. Dempsey says, ‘If the resin is too viscous, it won’t run through the carbon and you end up with fibres touching each other. Ideally you want them minutely separated.’ 

Then there’s the issue of compressibility, which affects the thickness of carbon structures. ‘Different additives in the resin will affect the compressibility,’ Dempsey says. ‘You can get a different layer thickness dependent on the resin’s characteristics. Generally, cheaper resins will be thicker. With a good resin, the carbon fibres can be microns apart. That gives you thinner walls for the same strength qualities, meaning a lighter frame. A cheaper resin leaves more material between fibres and layers.’

As Giant manufactures totally in-house, it’s been able to develop its own resins. Ward says, ‘We’re on our third generation of resin development now. The smaller details of the moulding and curing process are all down to the resin properties – the temperature it goes off at and the time the cure takes.’ Due to the wide price range of its carbon products, Giant uses two types of resin. ‘Our standard resin is used on all product lines except the Advanced SL products,’ says Ward. ‘For the Advanced SL, we use a nanotechnology additive. Nanoparticles increase the impact resistance of our frames by 18% with no negative effect on stiffness or weight. They cost a lot more though.’ 

An additional byproduct of the particles is improved wall compaction during the cure. ‘The nanoparticles allow the resin to fill the micro voids in the lay-up. The resin actually flows better, reducing the potential for voids and reducing wall thickness,’ Ward adds.

A resin’s role in void reduction is a key point in the structural integrity of a frame, as Dempsey explains. ‘Voids in the resin are holes that will collect stress,’ he says. ‘These are potentially failure points, and voids fail by blowing apart as the layers delaminate. You can still get delamination without voids, but you want to aim for minimal air pockets in the composite.’

In addition to load transferring, wall thickness and robustness, resins can have an effect on the ride of the bike. Dempsey says, ‘From a simple point of view, you can think of resins as a two-pack Araldite style product with a resin and a hardener. The quantity of the hardener used with a given resin can have a substantial effect on the ride quality. For a good bike frame, you need some flex within a cured resin to allow for the transfer of stresses between layers of carbon fibre. You can get this by using a stronger resin with less hardener. Clever designers can get a stiffer or more conforming structure for a given weight. You can’t rely on a resin for stiffness but, as an engineer, you need to be aware of the potential properties the resin can add to a finished structure.’

Resins are clearly important to the quality of the finished frame, so we return to the question of why we hear so little about them.

‘Resin is an allower, not a feature driver,’ says Dempsey. ‘Resin allows us to bond the different layers of carbon fibre together – for example T700 to T800 – to use the differing properties the fibres present to us. It’s a hard sell, and very difficult to spin, but the role they play shouldn’t be underestimated.’ 

Giant’s David Ward puts it more succinctly: ‘Resins are only a glue. They’re just not sexy.’

Heat of the moment

Given that most bike manufacturers use pre-preg carbon, their choices are limited in terms of using resin to affect the performance of a frame. But that doesn’t stop people looking for new directions, or from pushing resin and pre-preg companies to produce different products. 

Dempsey says, ‘We’re working to get partners to produce a resin that doesn’t go off at room temperature. One limiting factor with design is that as soon as you take pre-preg from its cold storage, it starts to air cure. It will never go fully hard outside of the curing oven, but it will “go off”. A pre-preg that allowed us to use a more complex lay-up process, and develop our ply book [see glossary, left] to the level we want, would allow us to get far more from our end result. That would be brilliant for us.’

One area where resins play a huge part is in carbon wheel manufacture. Here resins are key, not only to the structural integrity and stiffness of the wheel, but also to braking performance.

Lightweight’s Leschik says, ‘The weakest point of a resin is its temperature behaviour. Most resins have problems over 150°C. Over the last 10 years we have increased the temperature resistance of our resins threefold.’

Almost every cyclist will have heard a horror story about a carbon wheel failing on a long descent due to heat build-up, but what actually happens when brake pad meets rim? Leschik says, ‘Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Braking on a CFRP rim with rubber brake pads in wet or dry conditions is one such tribological system. The optimisation of this system for good brake performance is not possible without high-temperature-resistant resins.’ 

As with frame performance, it’s additives in the resins that add to the heat resistance, and to the price. One such additive is a ceramic – silica. While Aprire doesn’t make wheels, Dempsey understands the process: ‘Resins make a massive difference in a carbon rim structure. For instance, adding silica draws away a substantial amount of heat from the body of the structure and allows the airflow to cool the rim far better than with a standard CFRP rim. Copper would be a great additive as it has the capacity to draw huge amounts of heat, but there’s potential for sulphur to leach into the resin if moisture got in through any micro cracks. This would lead to almost certain delamination. Heat sinks – meshes within the resin – have great potential. This technology may well come.’

Lightweight’s Leschik also has huge faith in resin developments: ‘We are looking at the optimisation of the rim-braking rims. With smart resins we’re certain we can give the rider the same brake performance as discs without a single extra gram of weight.’

The hard truth

It’s clear that resin is an unsung hero of the bike-building process. It can affect the stiffness, robustness, weight, safety and price of carbon fibre products, so can we expect manufacturers to start waxing lyrical about the wonders of their sticky stuff? Probably not, because it’s still only one part of a complex system. Great-quality resin will not make up for poor-quality carbon fibre or uninspired construction techniques. As Lightweight’s Leschik puts it, ‘It’s the same every time: to cook a nice cake you need the right ingredients in the right ratio, made well.’

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