Strength with infusion epoxies versus high performance epoxies 1

[trenace]

Beginner question, planning well in advance of a carbon fiber autoclaving project:

I see a tremendous strength difference data between high-perfomance epoxies, such as various Resin Services products, versus infusion epoxies I can find.

For example, reported values for LR-200 are:

Tensile Strength, psi 28,176
Flexural Strength, psi 38,000
Flexural Modulus, psi 2,187,505

I don't know what the strongest infusion epoxy is, but for example the MAS infusion epoxy reports:

Tensile Strength, psi 10,200
Flexural Strength, psi 24,000
Tensile Modulus, psi 350,000 (Granting that flexural modulus and tensile modulus are not exactly the same, but values seem closely comparable.)

Am I majoring on a minor here, with the differences not being important because the carbon fiber lends essentially all the strength anyway?

There's certainly a strength difference between using ordinary epoxy and say vinylester, but is a point reached where further strength hardly matters, with typical infusion epoxy already at that point?

Or would a part be significantly stronger with the stronger epoxy, assuming equal resin content and equally well-done work?

Does anyone have a recommendation for a higher-performing infusion resin?

Thanks!

 

[ESPcomposites]

Tensile strength won't matter too much. Notched strength (holes/cutouts) will be affected more than unnotched specimens (generally).

The major changes will be in matrix dominated properties such as compression strength after impact, impact damage resistance, radius opening (create high interlaminar tension and shear stresses), delamination growth, etc.

Brian

http://www.espcomposites.com

 

[trenace]

Thank you Brian!

 

[RPstress]

What is LR-200? A quick Google doesn't find it.

Your first figures much look too high for an epoxy resin alone. The likely tensile strength is about half that and the modulus is about four times too high. That looks like short fiber reinforced or something, though the strength and modulus values don't really tie up for me.

The second figures look fairly typical for neat epoxy resin, although the modulus looks a wee bit low (this quite often happens with infusion resins for some reason).

An injection resin will typically be largely untoughened (toughening agents tend to interfere with its flow). This will make it relatively strong, brittle and with a high Tg/HDT.

The main exceptions to this are PR 520 and Cycom 977-20 (with this latter, the toughening agent is put into the fiber preform and the resin injected is an untoughened version of 977-2). PR 520 in particular rivals autoclave-cure prepregs for toughness, although it can take a great deal of experience to successfully inject it. Having said this, there are a bunch of newer injection resin systems being developed, which often have a 'veil' added to the preform which acts both as a tackifier (which helps bind together the preform layers) and as a tough final laminate interlayer.

In general, as you make a resin tougher, the Tg drops, and the strength can drop a bit. Older prepregs such as 977-2 had anything up to 30% by weight thermoplastic in them. The current fashion for very tough interlayer-toughened prepregs are certainly tough (e.g., Hexcel M91, Cycom 5276-1), but their wet Tg is not much above 120°C from a 180°C cure. A relatively untoughened epoxy will often have a wet Tg above 160°C.

Separating the bulk properties of an interlayer-toughened resin from the laminate properties is quite hard, especially for toughness. A bulk resin sample will have the interlayer toughening particles distributed differently from when it's in a laminate.

An interlayer-toughened laminate might have a compression after impact strength twice that of a laminate with a really old resin such as the venerable 3501-6 (my current company stopped using that back sometime in '90s when 8552 became available). The first interlayer-toughened commercially available material that I'm aware of was 8551-7, developed by Hexcel for the GE carbon/epoxy turbofan aeroengine fanblade, where the birdstrike requirement needed something a step up on what was then the state of the art.

 

[RPstress]

Oops, 'much look' should be 'look much'.

I've now found LR-200,

http://www.resinservices.com/custom.asp?CustomID=14&Productid=31

.

The LR-100 seems similar but has a claimed modulus of 1.4 Msi (though the tensile strength is similar). 65% of the modulus? And the claimed flexural strength if the LR-100 is virtually the same as the tensile.

The LR-200 CTE of 1.32 x 105 (1.32 x 10^-5) is a bit low for per °F.

The WB-400 resin claims to be 'unfilled' and looks more typical of a room temperature set epoxy (8500 psi tensile strength but no modulus given). The MCR-507 is 'hard carbide filled' and has a tensile strength of 91 ksi, more than all but a very few aluminum alloys. This goes with a flexural strength of 15.5 ksi (and compressive of 37.5 ksi)! Even for a particulate filled epoxy that 91 ksi is rather high, and the flexural should typically be between the tensile and compressive.

Their HTR-250 claims a tensile of 9500 psi, a compressive of 15000 and a flex of 42000 psi!

I don't understand Resin Services's claimed properties. I don't believe that the claimed properties can be for pure resin. It's possible that some sort of fillers are used, but the bizarre ratios of tensile, compressive and flexural strengths still don't make any sense to me.

Can anyone else shed some light on this?

 

[RPstress]

Not quite true about the flex strength being between the tensile and compressive. It's possible it can exceed the bigger of the two, but it seems quite rare. (Typically the beam fails either compressively or tensilely, but in bending things can be stronger than in nett tension or compression; less of the section is at the maximum stress and as well as yield effects statistical effects can help.)

 

[allcomposites]

Dear Trenace,

Thanks for pointing out a clever question that is frequently discussed when comparing autoclave to resin infusion processes.

Note that the resins you indicate are not the most appropriate for high-end applications. In fact LR200 is a laminating resin used to make composite mold for autoclave manufacturing. It has some filler on it (probably alumina micro-particles or silica-oxide). These are not indicated for impact applications (which is one of mayor issues in aeronautics).

You should compare resins as indicated by RPStress. Try RTM 890 from Cytec or RTM-6 from Hexcel. These are certified, world-wide used infusion resins.

AllComposites