Carbon nanotubes get a thumbs down

General Composites Discussions Composites Talk
#1

I had high hopes for carbon nanotubes after reading that they offered a 20x increase in strength and stiffness over carbon fiber.

I thought that even if the claims were exaggerated by 50% or even 75% then their use would be a no-brainer. The problem is that I simply can’t replicate these results, or anything close, by adding nanotubes to my cf parts.

Out of all my experiments, the only one where I was able to make anything that was noticeably stronger or stiffer than a similar sized cf part was where I replaced the cf with nanotubes completely.

There was no scenario where using nanotubes as a resin additive (for a cf part) made a meaningful difference. Even when I added a whole gram of nanotubes tubes to 30cc of resin (which was around 50:50 by volume), the increase in stiffness was minor (no viagra jokes!) once it was spread across a woven fabric.

I am not done experimenting with nanotubes but all future experiments will be focused on using them as a casting product (mixed with epoxy). I could see some potential for creating fast injection molded parts that are stronger and lighter than equivalent cf parts. At $250 per kg, it will only be for high end products though.

Has anyone here had different results? I am thinking that I could be doing something wrong and perhaps there is a special method needed when using them to compliment cf parts? Or perhaps not all nanotubes are created equal?

#2

It may be a marketing thing. But what type of testing was done to confirm your results?

Also when speaking to one of my reps they mentioned nanotubes are extremely bad when airborne. Trimming I mean, is that true or is that a rumor?

#3

Nobody knows how bad the nanotubes tubes are when inhaled as no testing has been done yet. They are rated as hazard because of the lack of info. From what I read, the small amount of testing that has been done did not show the same level of lung inflammation caused by things like asbestos. Still. I wore my mask when I poured them into the resin.

My testing was not particularly scientific. I was assuming that if a material was 20x stronger and stiffer then you would be able to feel the difference without any equipment.

My tests are simple things like resting it over two pivots and seeing how much weight it takes to make it bend in the middle.

I saw some differences but they were mostly too subtle for my tests and… more importantly… they were not repeatable.

While some of the gains might be considered statistically significant by a scientist, my customers are more practically minded. I.e. It would have to show a meaningful improvement in the strength to weight ratio to justify the extra cost.

I saw the most potential and obvious benefit when the nanotubes replaced the cf completely. I.e. They showed the ability to make lighter and stiffer parts which makes them interesting for high end rifle stocks where competitive shooters invest in the best gear and weight restrictions apply.

I only tested stiffness. I have no way or desire to test strength. For my applications, carbon fiber is many times stronger than it needs to be. It’s all about stiffness and thermal stability.

#4

I should also state that I think the shape has more to do with the stiffness than the material. I have been using 1/8" long carbon fiber micro tubes and I have seen awesome increases in stiffness. This is probably at the expense of some top end tensile strength but I don’t need as much of that.

There is an argument for stating that all tests for nanotubes should be vs carbon fiber tubes and not flat cf pieces.

#5

But how did you mix them into the resin? From what I’ve read that’s the secret sauce. As the nano tubes need even distribution in the resin and they tend to separate and clump. I had seen a premixed resin system that had the nano tubes mixed in some way that they stayed in suspension. Never used it though. I was wondering what it costs?

And there are different types of nano tubes… varying lengths and single and double walled. It’s really once it’s made into sheets that it will be more useful. Graphene sheets are available now, not sure what benefit it would have though.

#6

I only saw any noticeable benefit when I saturated the resin with nanotubes. I can’t comment on the effectiveness of techniques to evenly distribute the nanotubes in the resin as I haven’t tried them. My intuition tells me that it wouldn’t make a difference though, unless you use enough to notice.

So far, I have had more luck making stronger lighter parts using carbon fiber tubes. I have some that are 1/8" long x 1/32" diameter and the difference then make is enormous. They are a game changer for me.

#7

there was a company that was making the resin that had the nano tubes in it and in such a way that they remain in suspension and did not settle. I haven’t seen that product take off…

Zyvax also makes carbon nano additives as well as a pre preg with nanotubes.

http://www.zyvextech.com/arovex/

though it seems like the nano tubes mostly effect toughness though they do say it provides 35% gain in flexural modulus. Manufactures often quote numbers that don’t actually play out in real world testing though. I also wonder about the price for these sorts of materials. If it really did provide the increases in strength they claim, these materials would be in more use in aerospace… I would think at least?

As for using tubes, you use them in your layup? Or just as a tubular structures?

I’ve seen some amazing truss structures…

#8

I use the micro tubes in my layup for parts where I want high stiffness and reduced weight. Depending on how I use them, there could be a cost in extra brittleness but this is unlikely to be an issue for what I make. The tensile strength of cf is many times greater than I need it to be so it would be next to impossible for anyone to snap one of the parts by accident.

Where I need both strength, impact resistance and stiffness, I use a mix of woven fabric with the tubes.

There is just no comparison between the stiffness of a part made with the carbon fiber microtubes and one made with the same weight in woven fabric. The tubes are significantly less expensive too.

The downside is that they don’t leave a nice surface finish if used without a woven fabric being laid down first. If I use them on their own, I have to put an epoxy gel coat down first.

The reason I don’t have any faith in clever techniques to evenly disperse nanotubes in resin is that the numbers don’t make sense. If the nanotubes were really 20x stronger and stiffer (as they claim), you would need to use the same volume of nanotubes as you did with the cf to realize that benefit. That’s a lot of tubes… far more than they are using.

you could potentially choose to create parts with equal strength but less weight buy using 20x less but that’s still a lot of tubes. The nanotube resin would have to be more like a black paste as the resin is saturated with the nanotubes. The ready mixed nanotube resins I saw had so few nanotubes that they barely even changed the color of the resin. They certainly didn’t change the viscosity.

I was able to create very strong nanotube parts when I added enough to turn the epoxy into a black sludge but I have no reliable way of testing any potential improvements over using pure cf in a well designed part. Unlike the nanotubes, regular cf offers more opportunities to increase strength and stiffness with clever part design and / or different weaves.

I think there is a lot more room for further tests but I don’t feel like investing the cash in R&D right now. My initial testing just wasn’t exciting enough to warrant it.

#9

The tubes are supposed to be an additive to the overall part. I don’t know what quantity exactly but it should be an insignificant increase in weight. The zyvex page has some case studies that they show as evidence for increased properties. Sounds like it could be good if the materials preform as they say they do. I’ve never used them or heard much from anyone about them from real world use.

I’m not quite understanding what you do with the tubes? You use them as a core or filler in your parts? Like you lay down a ply of cloth, than spread the carbon tubes out over the part, then another cloth? And you’re talking about carbon tubes that are pultruded, like the ones used for kite rods or arrow shafts? That’s an interesting idea. I would imagine you buy these in bulk? They’re cut offs from another process or something?

As for the Nano tubes, they’re extremely light and short as well, not long enough to be used in place of fabric. From what I’ve heard they help to ad inter laminar connections when they’re in a Z orientation, which greatly increases toughness. I would imagine the effect they have works on a microscopic level acting as bridges between the open areas of a fabric? But I’m not sure exactly… I’m not an engineer that deals with that sort of stuff. They’re only currently an additive and not a sheet material, though people are working on such things.

I don’t think it’s a matter of having more tubes = more strength… at least that’s not what it seems like to me. As being such small physical pieces, it would make sense that they would increase properties that work on smaller areas, like impact resistance/toughness. As compared to forces that work over larger areas, like stiffness. Again, I’m not an expert, just thinking out loud.

I think you’re right though. Using shapes and laminate design is the main way to increase strength in a part. There are various materials that can also be combined to create a stronger part. The idea with the nano tubes is that they could be added in a very small amount by weight to see a relatively large increase in properties for the added weight. It takes a lot of testing to figure these sorts of properties out… I"ve been cutting many many yards of material that goes into test panels for the various testing that we do. Making a few anecdotal parts and bending them by hand or whatever is a difficult way to asses the claims of the nano tube manufactures. As we know, testing is very very expensive and cost prohibitive to most manufactures of carbon parts with slim margins.

There is a ton of room for testing, which is part of the reason why we don’t see these materials used commonly.

Thanks for your insight into these materials, I doubt I’ll do anything with them soon. Still I know the future is in these sorts of nano materials.

#10

You don’t need to be an engineer to consider the principle that if something is 20x stronger by weight or volume, you would only realize the 20x benefit by using the same weight or volume.

Steel is stronger than aluminum but if your steel is as thick as a hair and the aluminum is 1/4" thick, the aluminum will be stronger. I don’t know of any material that is hard to break in nano thicknesses. It is absolutely the case that more nanotubes = more strength. Just like how one layer of cf can be torn by hand and ten layers feels impossible to even bend.

Carbon nanotubes and the 1/4" carbon fiber tubes I was talking about are two very different things. The nanotubes I bought came as a fine black powder with no discernible shapes visible to the naked eye. The 1/4" carbon fiber tubes are far larger and each tube is individually visible.

In my testing, I wasn’t able to achieve any meaningful improvement by using the nanotubes as a minor additive. My testing wasn’t massively scientific but I have little use or interest in benefits that can only be detected with specialist equipment.

I know what the claims are about nanotubes. I read all the same stuff you did before buying them. That’s what made me want to try them. I just didn’t notice much benefit unless I used a larger volume and made the nanotubes the main even instead of just sprinkling some into my resin.

The 1/4" cf tubes were actually bought by mistake. I thought I was buying 2lb are regular chopped cf strands and instead, I received these rigid little tubes:

http://www.ebay.com/itm/1-4-Chopped-Carbon-Fiber-16-oz-454g-1-lb-Real-Authentic-Carbon-Fibre-/322401026153?hash=item4b10994069:g:6zkAAOSwbqpT5kKT

I thought I had been ripped off initially as they were unusable for my purposes. They were just too hard and inflexible, almost like they had already been cured. I ended up testing them as a reinforcement for a mold base and I was so impressed with the results that I immediately bought another 2lb.

The benefits of using cf tubes (of any kind) in the part is that they provide a lot of stiffness while the air gap in the center helps keep the weight down.