Stiffest epoxy resin?

General Composites Discussions Composites Talk
#1

Does anyone here have a view on which is the best room temp epoxy resin to use with carbon fiber if creating the stiffest parts possible is the goal?

I have noticed some difference in the strength and rigidity of my cf parts when I switch resins but I have mostly focused on the choice of fabric and the direction of the weave to increase stiffness.

I am interested to see how much the choice of epoxy brand makes a difference and how much stiffness is effected by the length or temperature of the cure cycle.

Adtech claims to sell the best resin but I guess they all do…

#2

What are you currently using? Room temp cure and then post cure? You’re wet laminating or infusing?

#3

If you believe the datasheet from AdTech, their 820 with 824 hardener has a flexural modulus of 2,200,000. That blows away most epoxies that I have seen which tend to have a flexural modulus of around 600,000. I just purchased a gallon of the 820 with 824 hardener. I am going to give it a shot as stiffness is my ultimate goal for my parts.

#4

this epoxy from ptm&w is the one that I used in a similar application to yours. (I can take a picture if you wana see the part).

http://www.ptm-w.com/technical-library/product-bulletins/Epoxy-N-Fusion%20Bulletins/PT5760%20Bulletin.pdf

The data sheet lists the Flexural modulus as 3,057,938 psi (21,083 MPa) with glass and 7,304,718 psi (50,363 MPa)with carbon.

This sort of thing is some what difficult to compare as flexural modulus is largely driven by the reinforcement. The pure resin alone has a flexural modulus listed as 475,118 psi (3,276 MPa).

I would imagine that the resin you’re using and have the data sheet for would list the atsm standard for the test and the reinforcement material as reference for comparison. The neat resin properties will be comparably low to the reinforced obviously. The resin has it’s main strength in compressive force vs. tensile. While the reinforcements will have the inverse.

Comparably speaking though… from the numbers you give to the ones from the data sheet i posted, the PTM&W resin has much higher compressive strength. Though before you go out and buy some resin, I’d advise that you call them up and talk to one of their technical sales engineers. PTM&W is pretty good about helping you find the right resin for your needs. They also formulate resins and have the capability to tailor a resin if it was absolutely necessary.

Surely there are other good resins available… the one I linked here is great though, has super long pot life, very tough, but does require a post cure.

#5

The AdTech pure resin alone has a flexural modulus (E) of 2,200,000 psi. More than 4x that of the PTM&W above. Looks like they used the same ASTM tests.
Compressive modulus and not compressive strength would be a factor, but the PTM&W data sheet shows it for a laminate, the AdTech is for resin only, so you can’t make a valid comparison from the supplied data. If the flexural modulus is high, the compressive modulus will be reasonably high also.

So, with all of the many other variables kept the same, the AdTech should produce a stiffer part.

#6

Do you have the sheet? I’m betting that the flexural modulus number is not pure resin…

Ok, I looked it up after typing a bunch of stuf… http://www.axson-technologies.com/sites/default/files/TDS%20-%20Marine%20820%20160524_1.pdf

I think this is the right resin?

It’s not a neat or cast pure resin sample for the felxural modulus testing… there is no given information as to the type of reinforcement or amount. The tests that have the asterisk on them are the ones done with resin only. This is why using the TDS to determine the best resin is a bit of a hit an miss type of thing… if they did the test panel with all ultra high modulus unidirectional fiber with a very low resin content, the test will look much better. You might be able to call them and ask about the specimen, they might be able to tell you so you have a better picture of what the test data reveals.

here is another tds for a resin sytem that I use for comparison on how different they can be: http://www.tencate.com/emea/Images/TC250_DS_012417_Web28-3802.pdf

this tds has a ton more information but not everything that you might want… This is why we end up doing test panel after test panel in aerospace. The design margins are pretty tight and making things wider will only end up in a heavier part than necessary… also when it comes to certified craft, this sort of stuff can carry heavy liability in the event of lives lost or damage done.

I’m not sure if I’m helping at this point or just throwing too much information out… lol

I guess in direct response, No the material your using would probably not give stiffer part than the pt5760 considering the 5750 lists the flex at 3,057,938 psi using glass reinforcement and 7,304,718 psi with carbon fiber. Both number being considerably higher than the adtech sheet lists. There are still many other resins out there that probably are better than the one I thought of off the top of my head. Though each have different strengths and weaknesses as well as price. Choosing what is the right material is one of the hard parts. Ultimately you use what you have or can get and go with that until it fails you in one way or another. You can see though how the reinforcement makes such a huge difference, in this example a 2x increase in flexural modulus by going from glass to carbon fiber. You could probably go even higher with ultra high mod uni.

For your part I would expect that you use much unidirectional? I would think the resin would have some effect for sure, but the reinforcement should be providing the majority of the stiffness. Maybe use ultra high mod uni? Also the geometry is going to greatly effect stiffness. Having a flat piece of uni that is 100 plys thick probably wont be as strong as using a C Chanel or box shape. The wing spars are like this and have such a shape but still have an expected deflection, inches, at max load.

It sounds like you’re trying to squeeze out as much as possible? How are you measuring deflection now? And what is your target? Is budget an issue? How many do you need to make?

#7

You are correct Sammy. The AdTech sheet is only the neat data for the line items with the *. I saw on the bottom that it said “testing conducted on neat resin” and made the assumption that it was all the data. I am going to give the AdTech a go and see how I like it. Ultimately, for me anyway, the stiffness is going to be dominated by the IM7. The Pro-set resins have some pretty good numbers.

As for the other questions, I will leave them for the original poster, Zebra to answer so as not to hijack his thread.

#8

Tensile stiffness isn’t dominated by the resin, only the matrix. The resin will have far less effect on stiffness except for compressive properties. The resin to fiber volume will also have a big effect- use as little resin as possible- shoot for 40% resin, 60% fiber by weight.

#9

You meant to say fiber instead of matrix.

#10

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/

#11

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.

#12

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.

#13

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.

#14

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.

#15

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?

#16

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).

#17

No, once cured the stiffness is the same , although with no accelerator it can take 2 days to fully cure.

#18

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.

#19

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!

#20

The best way to make high stiffness parts would be to use high modulus or even intermediate modulus fibre