Features Fabrication Innovations
Lighter yet stronger – a look at new composite materials in fenestration applications

Carbon fibre composite remains too expensive and slow to process for widespread use as a building material. But trends both on the supply and demand side are perhaps bringing it closer to feasibility. The material’s strength-to-weight ratio, corrosion-resistance and low thermal conductivity make it an intriguing possibility for very high-end projects demanding low sight lines, big glass and good thermal performance.

Meanwhile, processing innovations are making the raw material cheaper and the fabrication process quicker. We discussed the far-out possibilities of carbon fibre composites with Richard Collins, technology analyst for IDTechEx.

GC: In general, how do carbon fibre composites compare to other common materials used for framing and architectural glazing?

Collins: The general rule of composite materials, and particularly carbon fibre, is that they can be viewed as a metal replacement. So they have the best strength and stiffness-to-weight ratio in comparison to it. You do see examples of carbon fibre being used for some very bespoke structural building and construction. Same for glass fibre as well, but glass fibre is not the same quality in terms of strength and stiffness to weight. So that’s kind of the payoff that you’re taking there.

GC: In general, how do carbon fibre composites compare to other common materials used for framing and architectural glazing?

Collins: The general rule of composite materials, and particularly carbon fibre, is that they can be viewed as a lightweight metal replacement. They have the best strength and stiffness-to-weight ratio in comparison to steel, aluminium, and titanium.

In terms of the role it could play, if you compare composites versus aluminum versus vinyl, composites and aluminum are much more on the same page.

GC: What makes carbon fibre composites an interesting material for building construction?

Collins: A composite material it is basically a fibre embedded in a matrix. When people say carbon fibre, what they usually mean is carbon fibre-reinforced polymer. That’s a carbon fibre embedded in a polymer matrix. One of the main interests for building construction is the strength-to-weight ratio of these composite parts.

Now, what you’ve got is that the fibre has got a fantastic tensile strength. However, as tensile strength is only relevant in one direction this does not necessarily make it suitable for compressive and lateral forces. There are ways around that. You can stack them in different ways so that you can have layers of fibres going in X direction, Y direction, and other angles and you can build them up like that. Alternatively, there are lots of options in weaving and braiding.

So when you say strength-to-weight ratio, you really have to ask what do you mean by that. When designed properly, they are more competitive in terms of strength-to-weight, but there are these extra considerations because they are a fibre.

GC: Why have carbon fibre composites become such popular materials in other industries, such as automotive and aerospace?

Collins: One of the things that people don’t always necessarily realize about carbon fibre is that strength-to-weight ratio is one thing, but actually it’s stiffness-to-weight that is special. That’s what makes it a potential automotive replacement in structural parts; that’s what makes it so liked by the aerospace industry in all those roles. And if it that’s key for your particular large glass product, that stiffness, then, yes, it would be very useful again.

GC: Do you see it as theoretically possible to design, say, a window frame made of carbon fibre composite that would structurally stronger and be able to bear larger glass panels with fewer mullions or other supports?

Collins: Yes. If we consider the specific strength or specific modulus, carbon fibre is about five to six times better than aluminum alloys as a ballpark figure. But again, that is in one direction. I’m sure with a frame you could design that appropriately.

One of the problems that they have for moving parts, like a suspension upright, is when you suddenly get a force from a different direction than what you designed for it could be devastating to your part. But, for a big window, you’re not really going to have too many unexpected forces. So you should be able to design it appropriately, would be my guess.

GC: How are carbon fibre composite items made?

Collins: Making composite parts is one of the biggest problems, particularly making them with a good degree of automation and a good degree of speed. That’s where there are a lot of innovations going on. What you effectively have to do is buy carbon fibre from some of the big chemical manufacturers. Toray are the market leaders. What you get is something that they call a “tow” wound on a bobbin – that’s thousands of individual carbon fibre strands. It looks almost like a thin black tape. And you lay all these down and usually stitch, weave, or braid them together to give a fabric or mat of just carbon fibre. Dedicated companies or the fibre manufacturers themselves usually provide these fabrics or mats. And then you have to infuse that with the polymer. So, you can infuse that in a variety of ways. In the most basic traditional way, you pretty much do it with a paintbrush. So you can just get your resin, paint it on there, wait for your resin to cure, and you’ve got a part.

Now, there’s a lot more sophisticated stuff. A more common technique is where you’ve got almost like an injection moulding process called resin transfer moulding. And then you got different types of compression technologies as well as progressing into more robotic layups and things like that. It’s a really complicated process.

GC: Can you imagine how a manufacturer might go about producing something like aluminum lineal out of carbon fibre?

Collins: What they typically do is make what they call a preform. So you would cut your fabric into shape in advance of then injecting it with the resin. Alternatively, there’s something called a pre-preg. This is where you have a fabric that already has the resin in place, but the resin’s not cured yet. So then you cut that fabric into your shape and then all you have to do is heat it and you’ve got your part. So, your milling process that you would traditionally do in a metal scenario isn’t quite the same. It’s more literally cutting a fabric and getting it in the right shape before you turn it into a solid part.

GC: You mentioned things are happening to make production of carbon fibre composite parts more efficient. What things are those?

Collins: It’s in the actual processing. Because of the automotive industry and others getting involved, they’re obviously so keen on speed. So, there’s lots of innovation around getting that processing time really fast and getting that turnover because, as you know, in automotive you’re looking at maximum three minutes a part – any more and you’re wasting money and time. So, there are lots of examples of that and that’s a step they are taking in that direction. But then you’ve got the associated industries. So if you go all the way back to making your carbon fibre then you might be turning it into a fabric. You might be weaving it, you might be pre-impregnating it. There are several steps, but then to make the actual final part they do try to make it as fast as they can.

Richard Collins

GC: What are some of the hurdles to using carbon fibre composite as a building material?

Collins: Really the thing holding it back in a lot of industries and particularly in a lot of, let’s say, main industries, such as building and construction – what keeps it in the realms of premium vehicles and aerospace – is the cost of the pure fibre is expensive. To make the carbon fibre itself is pricey. So, when you’re buying that off a chemical company, you’re already spending a lot. It depends what brand you buy, there’s almost a high-quality and a low-quality one, something they call a light tow and a heavy tow. So, the heavy tow is a slightly lower quality. I mean, it’s still very good. And that, at the moment, is in the order of about $13-14 dollars a kilogram. So, quite a lot higher than aluminum. And that’s just the fibre. That’s the thing that’s limiting it.

Now, what they say, typically, in the industry is there’s this magical price of $5 a pound. That’s about $11 dollars a kilogram. And they say when you hit that point, you will be accessing much more widespread industries. And there is hope and there is a lot of research going on to making that happen. So one way you can reduce the overall cost is in the processing. The way that you make carbon fibre is very energy-intensive and very slow. So, you can switch the process there. Alternatively, you can use a new precursor. So, the actual initial polymer that you use to make that carbon fibre, if you switch that precursor to something else that is, say, easier to come across, then you’ve really reduced the cost, as well, on that front.

So these do give it options. It’s not going to be quick. I forecast the cost of the heavy tow to be coming down to around the $12 dollars per kilogram mark within the next decade. But for the higher quality one it will only be coming down to around $16 a kilogram within the next decade, unless there is a big stepwise change in the use of precursors that could bring it closer to that magical $11 per kilogram level. Even then the lower the better. But even when I say magical, there are lots of limitations that come with this, holding it back.

GC: That certainly makes for a very expensive building material. Are there any benefits that could make up for such a high cost?

Collins: One point I should mention about carbon fibre, that maybe doesn’t get mentioned enough, is that one of the best things about it is its corrosion resistance. Because that’s a huge problem with metals. Even if we take the most fundamental steel-reinforced concrete, you’ve got a lifetime on that because your steel will corrode over time. And, of course, there’s lots of good chromium platings for steel – and many aluminum corrosion resistance solutions. But fundamentally you save all that by switching to carbon fibre. Corrosion resistance just basically stops being an issue. Now, there’s a lot of other issues, but that is something that is in its favour and perhaps doesn’t get the nod it deserves.

Other potential benefits are related to the coefficient of thermal expansion and the ease with which a lightweight material can be transported and installed.

GC: How thermally conductive are carbon fibre composites?

Collins: They are not thermally insulating, but they’re not very conductive. They’re definitely lower than a metal. So I think they are perfect, because they have some conductive transmitability, but they are lower than a metal. It’d be a lot closer to vinyl, a lot further away from the metal and give you some advantage on that front.

GC: Would you be able to make carbon fibre lineal with a thermal break cavity inside it, similar to a vinyl part?

Collins: It’s a good question. You can make hollow composite parts but, again, it’s more complicated because what you have to do is you have to have use a specific mould (what the industry call tool) that can be removed after the curing process.

So there are examples of this but they, again, add different complications and factors. The best being dissolvable tools, and 3D printing actually plays a role in this. But I think the most likely solution for your world would be making what they would call a sandwich part where maybe it wouldn’t be hollow but it would have a foam or honeycomb structure in the middle. So you could have it with a variety of different types, like a PET foam or something in the middle that could both add extra stiffness and also give you some properties, like vibration dampening, sound dampening, thermal, electrical and maybe some moisture resistance.

About Richard Collins
Dr. Richard Collins is a technology analyst for IDTechEx, a U.K.-based technology consultancy with offices in the U.S., Germany, Japan and Korea. He has a background in polymer science and leads IDTechEx’s research into lightweight materials.