You meant to say fiber instead of matrix.
Soller Composites sells the Adtec 820 that has carbon nanotubes mixed in. They claim a 25% increase in stiffness. http://www.sollercomposites.com/composites/Epoxy.html#nano
You might try one of their small quantities and see if it is worth the added cost.
SS/
I currently use a range of different resins and processes. There is always at least 6 or 7 types of epoxy on my shelf but I am specifically thinking about room temp plus post cure processes. It doesn’t matter if it is done by standard wet lay-up or infusion etc. I can do either here.
The main reason I am asking about people’s experience instead of just reading the data sheets is because I have never found much correlation between the stated data and the actual properties of the resins. Sometimes the resins with the best numbers turn out to be the most brittle and the least stiff.
There is obviously a lot of variables which dictate the stiffness of a part so the only way most of us will notice a difference (if there is one) is from making the same part with different resins without varying anything else too much.
Anyway, it doesn’t sound like there is currently a consensus on which is the stiffest room temp resin so it maybe a little experiment is in order. The choice of resin either makes a huge difference to the end results, or… it’s all about the reinforcement and you may as well just by the cheapest epoxy possible.
The stiffest resin I have used is axson 2020. However it is not cheap. It is an interesting system as it has an accelerator as well as a hardener. 7 per cent accelerator gives a 25 min pot life and no accelerator gives 2 hr 20 min pot life and varying times in between. Quite useful.
I think you got it right on the head… it’s all about destructive testing. This is one thing I find to be annoying with composites… you gotta do endless testing of materials and process. Without this data there really is no way to know how the various materials will operate in real world conditions. For aerospace, this is obviously extremely important. This is why we end up making endless amounts of test panels to verify, test, and ascertain the properties.
The data on the sheets is not all that useful in reality… maybe only as a rough gauge to decide between one system or another but not enough to actually design a part on.
As you say, there are a lot of variables… too many. It’s also hard to tell when something that seems minor, is actually making a large effect, and vice versa. Seems to me like there are lots of ideas about what does what or how it effects something but, often they’re purely anecdotal. When ever one of these conversations comes up, I like to see the data that supports it. With no data, it’s not engineering… But as you say, to test takes much time and material. And might lead down a tangential path to no where.
I often tell folks, “make a some test panels.” You can setup something to do some testing. Keep your process consistent and the results will probably be useful.
I was about to say the same thing but you beat me to it.
The way I test resins is usually by curing some on single thin strip of cf. If there is a large difference I can usually feel it in how easily it bends and tears.
If I want to get (very slightly) more scientific, I cure 5 layer strips (laid in multiple directions) and see how much weight it takes to cause a visible bend. This lets me test multiple products under the same conditions (I.e. Temperature).
If I was making any type of vehicle (land, air or sea), I would want far more in depth testing.
I use the same methods to test cf fabrics and different weaves too (all using the same resin). This is why, when I aim for maximum stiffness in a cf part, I typically prefer to use a 4x4 twill over a 2x2. I find a single layer of 8.4oz 4x4 to be noticeably stiffer than 2 layers of 6oz 2x2.
I think that the important of finding the stiffest resin is that it is essential if you want to make parts with the best possible stiffness to weight ratio. That’s the implication - more stiffness for the same weight of resin. Then you need the right cf materials, post cure schedule and part design.
That is useful. This is the reason I like Composite Envisions basic laminating epoxy for testing designs. It is available with 4 speeds of hardener ranging from one that hardens while you are still stirring it to one that lets you grow old while laying it.
I like being able to control when resin will gel and get tacky so I can choose the right one for the size of part.
Do you find that using the accelerator has an impact on stiffness and / or surface finish quality?
Hi Zebra, in my experience, 72 is a good working temp. If your shop is 72 when you go home and it goes down below 60 at night, no good! In fact I have made castings in those conditions that were essentially gummy bears thanks to cooling overnight! Steady optimal temps are very important! (Otherwise epoxy turns to gummy bear) Last January, I upgraded insulation in shop to control humidity and temp (I also work with fragile woods).
No, once cured the stiffness is the same , although with no accelerator it can take 2 days to fully cure.
In theory, a lower temp should only effect the curing time. I.e. If it’s too cold, it might feel soft for a week instead of 24 hours.
I usually post cure but just because of time constraints, I usually leave all room temp parts to cure in my boiler room which is always well above room temp. It’s not oven hot in there but it’s definitely at least as hot as the inside of a car after it has been left in the son for a while on a hot day.
For me it’s important to get parts to a high level of stiffness asap as most of them are long and thin with straight sides which makes them prone to warping if I am not diligent about temp control.
I am liking this Adtech 820 resin! Good job adtech.
Having a room temp epoxy remain usable up to 180 degrees without any post cure has some real advantages for parts that are too large for my curing oven, or for people that aren’t set up to heat cure.
Some of the cheaper resins start to lose properties at temps as low as 120 degrees so parts may be unusable in summer. It could easily get hotter than 120 degrees if left in your car on a hot day…
They also claim that it is UV stable so it won’t yellow in the sun etc. we’ll see. I hate clear coating and I like using less effort so it will be great if it’s true!
The best way to make high stiffness parts would be to use high modulus or even intermediate modulus fibre
I can’t argue with the difference the choice of fabric makes but it is logical that the stiffest parts would use a combination of the stiffest fabric, the best design from a stiffness perspective, the right orientation and the stiffest resin.
High modulus fabrics are apparently more brittle but the biggest issue with them for me is finding them to buy. I have only once ever found high modulus twill weave fabric available at any of the suppliers I use. Do you have a source for it in America?
Hi mod fabrics are going to be Unidirectional generally… I do believe that they’re not very good for weaving as the crimps will make them much much weaker. If you were using prepregs, i have a load of high mod stuff around. I’ve never seen the high or ultra high mod in a woven, but surely it must exist if it’s feasible?
You would just use the uni in you layup.
I saw this picutre of a music instrument part, this is what I’d do for more rigidity
take a flat piece an try bending it on it’s side and see how much stiffer it is.
They exist as woven fabrics. I have some that I bought a while ago. I just hardly ever see it available to buy anymore. I believe the seller was liquidating some kind of industrial operation.
High modulus fabrics definitely makes a difference to stiffness and sometimes I prefer to trade some impact resistance for greater rigidity. Carbon fiber is usually many times stronger than it needs to be for my applications but I can always benefit from greater stiffness.
I guess it doesn’t really matter what style of fabric it is because you can always use a more attractive style for the surface layer for most applications. The reason why it matters for me is that drapability is a significant concern for some of my parts.
I have found that using a 4x4 twill is good alternative. Even the standard modulus fabrics produce really stiff parts (much stiffer than most 2x2 twill) and drapes easily over sharp bends. I gave up looking for high modulus fabrics due to limited availability a while ago.
I would find less use for unidirectional high mod fabric because it usually means losing the stiffness to weight benefit as I have to add extra layers.
“I would find less use for unidirectional high mod fabric because it usually means losing the stiffness to weight benefit as I have to add extra layers.”
what do you mean by this? The nice thing about the unidirectional is that it allows you to tailor the fiber direction very precisely. Unidirectional is generally thinner and has much higher fiber volume as well, so it’s also generally lighter and stronger than a part made with fabric. Obviously it’s more time to layup as, for everyone twill ply, you’d need two uni plys to make it equal in directionality. Also as you know, Uni is a bit more difficult to work with.
I’ve also used an 8 harness satin weave cloth once, it was exceptionally rigid. Not sure if it was the fiber itself on that fabric, or the weave. The harness and twill weaves tend to be stronger due to the fact that they have less crimps. The harness fabrics though, are a little more of a pain as they need to be flipped over as they’re laid down to ensure good nesting of the tows.
I mean that uni is great where you want to make parts strong against forces in specific and predictable directions but if you want multi- directional strength you have to add more layers to cover it vs fabrics which have bi-directional strength to start with.
As for the thickness, I have had uni fabric in a variety ranging from fairly thin to fairly thick, like my other fabrics. It’s what you order.
Don’t get me wrong. uni has a place and is great for certain things. It’s just not ideal for what I make usually. I do use it where it’s appropriate.
I am experimenting with carbon nanotube resin this week. If I get the results promised on the tin, I’ll be changing a lot of things. Apparently it increases stiffness by 30% without changing anything else. I am kinda curious what it does if parts are made of 100% carbon nanotube (with resin).
The price of nanotubes has come down so much that I am surprised we haven’t seen more use of it. The hype is that it is stronger and lighter than carbon fiber’s.
Yah I hear you about uni… it’s a pain to produce and often times not as durable as a woven part but, also has it’s strengths. 
I just know for high modulus parts, It’s great because it can be tuned for strength in specific directions. I have a ton of it sitting around and still haven’t put it to use 
Nanotubes are cool. Is this the reason that claims to give like 10-20% better numbers? Nanotubes will be in our future soon, I hope.
Yah it’s stronger but, making it in a useable format for aerospace parts is a bit off. They have some larger production machines and are ramping up to try to make it more available. I also have seen that graphene sheets are now available as well.
Please let us know what you think about the nano tube resin. It sounds like it’s a good deal if it does give much much better properties with no other changes.
Interested in your results with nano tubes. I have a freind in the company I buy my resin from ATL composites. They did extensive testing with nano tubes and didn’t have any outstanding results.
They where supplied to them buy a leading german composite company… The company where very dismissive of ATL’s results from there lab using there nano tubes.
So ATL sent them some of there resin , so they could do there own tests using ATLs resin. And see for them selves.
The fact of the matter was , a very high quality resin is stronger than a crap resin filled with nano tubes.
And adding nano tubes to very high quality resin doesn’t make much of a differance compaired to the cost…added so it not worth doing for a small % increase. Concidering a slightly high post cure temp will add more stiffness / strength.
This is 100% the reason for nano tubes have come massively down in price and are not taking to composite world buy storm.
High quality resins post cured at high temps or pre pregs resins are stronger at this piont in time.
The composite evolution is a wonderful thing , so much is always happening in the industry it’s great to be part of it. No time to be bored 
Tim
I think there is more to it. I would certainly agree that there could be at least a 30% difference in stiffness between parts made with quality resin and parts made with low quality resin. That difference is enough to eat up the benefits of adding nanotubes.
Any test has to be done using the same resin and the same everything else. There are too many other things that effect stiffness to do it any other way.
My findings are limited so far but what I have seen is that the amount of nano tubes you add to each ml of resin makes a huge difference. My guess is that if ATL were not able to achieve any increase in stiffness from using nanotubes, it was most likely because there weren’t enough in the resin.
I find it very easy to imagine that resin makers would be a little stingy with the nanotubes in order to increase profit margins. I bought my nanotubes separately so I can decide how much to add (if any).
It’s the same issue with regular carbon fiber strands. If you add a small handful of cf strands to a gallon of resin, there will be no meaningful stiffness benefit. If you do a 50% resin / 50% cf mix, there would be a huge difference. Similarly, there is going to be a huge difference between adding 1/2 gram of nanotubes to a part the size of a car hood and using 1/2 kg.
To put it into a little more perspective for people who haven’t seen nanotubes in person, they look like a black powder. The premixed nanotube resins I have seen are clear and they advertise that it makes no difference to the aesthetics of the part…
Carbon fiber strong but… if I made a sheet of cf that was too thin to even see, how strong would it be? Even if carbon fiber was 5 times stronger, you could still snap a part easily if it was so thin. Thinking logically, how much stronger than cf would nanotubes have to be to make a 30% increase in stiffness from a layer that is too thin for the human eye to see?
I think that the price of nanotubes have come down mainly due to more efficient manufacturing processes with increased scale. The same reason why carbon fiber has come down in price too.