Moving Bubbles Around More Efficiently Is The Future Of Composites
Hey there, welcome to the February feature issue of the newsletter. This month I’m diving into composites and an interesting start-up trying to change the industry by well, moving bubbles around. CPS is back as my sponsor for this issue and the next seven.
I think that climate change facilitated by human driven carbon will be the biggest problem facing the human race for the next 20 years. I’m not 100% sure we can stop it, but I do think we can mitigate the worst case scenarios. Mitigation will require scaling expensive and known technology to drive lower costs to enable widespread adoption. Digitization will play a significant role here, but I’m not talking about ordering my dinner online, getting the wrong order, and then just living with the wrong food and a refund.
If we want to avert the worst case scenarios of our planet’s future it’s going to come down to advances in chemistry, mechanical and chemical engineering, and new ways of thinking about what most consider to be mature businesses.
If we look at where our carbon emissions come from with respect to “end markets” the US EPA has a really simple graphic based on data from 2019. Essentially, a third of all greenhouse gas emissions come from the transportation sector. Electric vehicles will definitely have a role to play here in drawing down those emissions, but we may be trading one problem for another (not as immediate) with respect to lithium and cobalt refinement. If you are unfamiliar with the issues around lithium and cobalt mining try here, here, and here. Essentially, there are a bunch of externalized costs around mining that make lithium batteries possible, and if you want them to be cheaper, then that cost has to come from somewhere.
Why do we want to make all of our batteries out of Lithium?
The atomic mass of Lithium is 6.941 and is one of the lightest electrolyte materials we have for batteries. Going to a higher mass element such as iron (55.85) could enable either significantly lower costs or higher energy density from an abundant element. Alternatively, we could also extend the range of our vehicles massively to reduce the total amount of cycles that our batteries go through. Essentially, the more a battery is run towards zero, fully charged, and then run back down decreases the overall capacity of that battery to hold a charge. Here is a more in-depth explanation. It’s difficult to increase the energy density of the lithium based batteries we do have now, but what if we could decrease their overall workload?
Future Of Transportation
The future of transportation is going to come down to a few things including but not limited to power sources, range, load capacity, and safety. There are a lot of advances needed in getting us away from internal combustion engines including energy storage technologies and rethinking how we move people, raw materials, and finished goods around the world. One area that I think doesn’t get enough attention is the strength to weight ratio of modern materials. I’m using an Ashby plot of Modulus vs Density (log scale) to show how composites can have similar strengths to some ceramics and metals at a much lower density. If we want strong and lightweight materials for our modes of transportation then composites are the material of choice.
A successful use of composites was recently seen in the Boeing Dreamliner. The Dreamliner is considered the most fuel efficient aircraft in the Boeing fleet and it consists of about 80% composite. If you are new here and have no idea what a composite is, it's a marriage of polymers and a reinforcing fiber such as carbon fiber or glass fiber. F1 cars for instance utilize a lot of carbon fiber composites.
The polymer phase is often referred to as the matrix and the high strength fibers might be known as the reinforcing phase or just as filler. If we think about the matrix and reinforcement phases, each material on its own isn’t really that useful for doing structural material applications. If you built a building or a car out of 100% glass it would be really brittle. When anything ever accidentally hits your glass structure there is risk of developing a crack which could lead to a fracture and catastrophe. If you built a car or building out of 100% polymer it would be quite weak and thermally challenged. The marriage of the two materials is the key.
The Dreamliner uses a lot of composites, but so do wind turbines, telecom, utilities, sporting goods, infrastructure, and even houses (video above). The failure of a composite in any of these applications would have different outcomes and the more high value the application the more certainty we have to have around understanding control of failure modes. If your golf club breaks you’re having a bad day, but if your airplane breaks?
In aerospace applications, like the Dreamliner, the fuselage of the airplane utilizes a lot of composite, and to fully polymerize the matrix around the reinforcing fibers many of the reactions are done in autoclave ovens. An autoclave is essentially a big pressure cooker with the ability to apply vacuum. An autoclave uses heat to drive the reaction of the polymerization to completion, it uses vacuum to remove gasses from the composite, and it uses high pressure to minimize the size of any remaining gas bubbles by crushing them or driving gases into solution within the polymer matrix. A full cure cycle might take 4, 6, or 24 hours to complete. Further, the need for an autoclave adds a significant amount of cost to producing composites. Not exactly something that can easily scale towards mass production and it’s all because of attempting to get rid of gas bubbles.
Why do we care about bubbles in composites?
If you are wondering about these bubbles and where they come from or why they are important then you are probably not alone. Next time you are fixing a broken mug at home with some 5 minute epoxy resin look at how it mixes together when you use it. Mixing two viscous liquids usually results in bubbles of trapped air and these are the types of bubbles that when introduced to a composite that can result in fractures. This is not the only way that voids or trapped gasses can be introduced to a composite, but their significance comes back to structural integrity.
Control of trapped gasses or voids in a composite is critical to ensuring the composite maintains structural integrity during service. If we think about a composite being an inch or two thick and a void that is maybe a quarter inch in size then that is a weak point and a definite pathway to failure. Think about having a random void in the steel frame of your car frame and hitting a pothole at 80 miles per hour. Autoclaves are trying to mitigate these weak points by controlling heat and pressure in either removing those bubbles or compressing them to be very tiny.
Making a few hundred Dreamliners per year works for an autoclave composite material, but if we want to dramatically reduce costs for composites while maintaining the high confidence level of strength and reliability we need new ways of removing trapped gasses on voids prior to polymerizing or curing our matrix resins.
There are a bunch of different ways to make composites. Here are a few:
Pultrusion
Resin Transfer Molding
Vacuum Assisted Resin Transfer Molding (VARTM)
Pre-impregnated fiber mats otherwise known as “Prepreg”
Extrusion and Injection Molding
Pre-impregnated composites offer a way for composite producers to essentially out-source their upstream production of mixing the resin and getting it next to the carbon or glass fiber. Imagine you are Boeing or Airbus. Your goal is to make really reliable airplanes. Same with a golf club manufacturer or an automotive manufacturer, making golf clubs or cars is the name of your business and infusing carbon fiber with resin seems like a job for a supplier. This is where the strength of a pre-impregnated or prepreg composite really shines.
Pre-impregnated composites are really exactly how they sound. Fiber mats are pre-impregnated with a resin matrix, such as an epoxy resin, and then partially cured or “B-staged” before getting shipped to the final customer. This enables the final customer to arrange the composite exactly how they want, perhaps in contact with another composite prior to fully cureing the composite in an autoclave.
Right now to utilize composites, many producers have to use an autoclave, but what if they didn’t have to?
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Apogee
Apogee’s mission is to get prepreg composites out of the autoclave.
I spoke with the founder of Apogee, Bill Edwards, about out of autoclave prepreg composites at the end of 2021 and I’ve always viewed composites as a sort of unique polymer chemistry problem. I think about the resin matrix in terms of viscosity, fracture toughness, pot life, and cure cycles and the challenges. Resins are only half the problem though.
The other half of the problem is what Apogee is trying to solve by providing a new route to make prepregs and revolutionize composite manufacturing.
Bill told me that for aerospace applications high fiber to resin ratios are needed. Any excess resin doesn’t really provide benefit, but it does add weight which isn’t desirable. Prepregs are one of the best ways to get high fiber content in composites, with better control than vacuum assisted resin transfer molding (VARTM), but the problem of getting the gasses out of the composite has been a big problem.
The benefits of VARTM are that the resin is pulled through the fibers with a vacuum, gasses are almost fully removed prior to the resin impregnating the fibers, but getting the part fully impregnated before the resin gels or vitrifies is the challenge. Further, having too much fiber in the vacuum bag during impregnation can hinder the flow of the resin so achieving full impregnation with high fiber content can be very difficult for composites made with VARTM.
We can think of vitrification of a thermosetting resin as a stopping of flow prior to formation of a three dimensional network, also known as a gel point. When a thermoset vitrifies additional heat can help to get it to flow again whereas when a thermoset has gelled it’s not going to flow. This is because as thermosets cure they increase in viscosity and then they eventually gel. All of the reactive groups in an epoxy for example need to find their counterpart (epoxy+amine) to fully react. If you want a deeper dive into how epoxy resins cure go here. Also, there is a whole class of polymers out there called vitrimers, but that’s a bit off topic.
If we think about a 4 foot long composite in traditional prepreg technology, Bill described getting bubbles and gasses out of that composite by pulling them through the length of the part as opposed to vertically through the surface. Going the length of the composite in a prepreg can be difficult and time consuming because the resin is curing and that battle of removing all of the bubbles may never never be won. This is why autoclaves get used, any remaining bubbles get pressurized into really small bubbles or driven into solution within the polymer matrix. Bubbles in a final composite are defects and defects can lead to weak points and weak points lead to fractures. Fractures are bad.
Apogee’s process makes bubble removal easier by enabling bubbles to go out the surface of a composite part as opposed to edges. Instead of the bubble traveling the length of the composite it might only need to travel fractions of an inch out the surface–often a 98% reduction in distance traveled.
Talk about an efficiency gain.
Getting a prepreg to be bubble free before going to the final curing stage means being able to cure parts outside of an autoclave. Reducing the need for autoclaves means lower composite costs and lower costs in theory mean more widespread adoption of composites. Apogee aims to reduce CAPEX requirements by 85% for composite makers when installing new composite manufacturing capacity.
Apogee’s long term goal is to be supplying aerospace companies, but their near term customers are actually sports equipment manufacturers. Carbon fiber is already used in golf clubs, bicycles, hunting arrows, and even footwear, but Apogee could help reduce costs on these composites by delivering higher quality prepregs that don’t require an autoclave to cure. These applications need high reliability and the time to market is significantly faster than the time it takes to get through the aerospace vetting process.
Bill is currently the only employee of Apogee (at least when we talked in December) and the company just closed on a round of funding that will enable the company to build a pilot prepreg manufacturing location in the great Los Angeles area. A 12” prepreg machine will be Apogee’s chance to show progress and there is potential for a 60” machine in the future provided they hit certain milestones with their investors.
Bill is also interested in pursuing composite repair applications with Apogee’s prepreg technology. If we think about wind turbine blades or military composite needing repairs Apogee could be offering a high performance lower cost solution than what is currently available. Bill also sees value in going after actual wind turbine blade production with Apogee’s technology and it could be the future of how even larger wind turbines are produced.
I asked Bill what it would take to get composites at scale into automotive applications based on the problem I outlined in the beginning. Imagine extending the range of EVs by making super lightweight and strong cars and then pair that with the advances occurring in batteries. He told me the cycle time of automotive parts needs to occur in minutes while carbon fiber epoxy composites typically take hours. Essentially, we need new snap curing resins with the performance of epoxy resins. It’s a good polymer chemistry problem to try and solve.
For now, Bill is focused on building out Apogee’s first pilot site and getting a team together. If you are interested in contacting Bill to trial their materials or take part in the next round of fundraising he can be reached at BillEdwards@apogeecomposites.com.
Great article! Your examples helped a non-chemist like me really understand the material.