Firewall seperate from seat?

[Nate Notta]

How many teams use the seat as the firewall... or similarly don't have a seatback, just their firewall with padding? Last year we had a seperate seat and firewall but it appears this isn't necessary?

Also, the rules on firewall are something like "non-permeable, fire-resistant..."... so what has passed for fire-resistant in the past? fibreglass?

Thanks,

Nate

[V2-iacoto]

hi nate,
we use the seat as the firewall.
It is a alluminium sheet with a fire resistant (amianthus fiber) sheet glued on.

BY

tom

[Michael Jones]

Fiberglass with standard epoxy really don't cut it. The glass tensile temperature (Tg) is in the mid-100 degrees F - at that temp, the resulting composite becomes very rubbery and malleable. West System 105/205, for example, has an ultimate Tg of 142F, which is surprisingly easy to get up to. I wouldn't want to sit on that.

You can, however, get heat- and fire-resistant epoxies that increase Tg and are fire-resistant. Combine that with Kevlar walls (Kevlar being far superior with respect to both fire resistance and penetrability) should be defensible. A few Google searches should set you off on the right direction there.

[Frank]

so many teams have used their CF/epoxy seat as a firewall (including us)

i don't think it's good, but its light

I'd prefer to see phenolic resin used
i haven't ever seen a bottle of it... apparently it lets of formaldehyde gas when its curing (toxic)

[James Waltman]

Actually Michael, the Tg is the Glass Transition Temperature. The Tg is the temperature where a material starts to flow. It is a common term for polymers. For example polyethylene, ABS, PVC, and polypropylene also have a Tg. The matrix material is commonly a polymer (epoxy or polyester) and these generally have a low Tg. Fibers made of glass (aka fiberglass) also have a Tg but it is very high. Signs that the material has reached its Tg and is able to flow are, as you said: "the resulting composite becomes very rubbery and malleable"�. At 150 degrees F you have reached the Tg of the resin. You are nowhere near the Tg of the fiberglass and the tensile strength of the individual fibers is essentially unchanged.

As a general rule of thumb the Tg of an epoxy resin will be within 50 degrees Fahrenheit of its cure temperature (of course there are exceptions). So if the resin is cured at room temp you won't get a high Tg even if the resin is capable of it. You will need to cure at an elevated temperature or post cure.

Polyester resins are usually flammable and not generally chemical resistant (this could be a problem with the non-permeable part of the rule). Epoxy resins are generally more chemical resistant and are available for higher service temperatures. Some epoxies are good up to 700 degrees F but these are very expensive, very difficult to work with and require an elevated temperature cure.

Kevlar (aramid) does not have a very high service temperature. Kevlar 49 has a recommended max service temperature of about 300 to 350 degrees F. Our headers are very near our firewall and our oil tank touches it so 350F is not high enough for us. It may be for some teams with different packaging. Fiberglass has much higher service temperatures and generally better chemical and solvent resistance than Kevlar. Remember, Kevlar is a polymer.

Frank,
In our Into to Polymers class one of the labs is compression molding phenolic resin. We even get the chance to try different reinforcements in it. We use a novolac resin. A novolac is a partially cured resin. Ours comes in pellets. Phenolics are common in passenger airplanes because of their flame resistance. Phenolics give off three things during the cure:

1. Formaldehyde – a nasty chemical
2. Ammonia – a nasty chemical that actually acts as a catalyst.
3. Dihydrogen monoxide – by far the most dangerous. Dihydrogen monoxide is colorless, odorless, tasteless, and kills uncounted thousands of people every year. To get more info check out this website http://www.dhmo.org/

We have a lot of composite parts on our car and have significant experience with composites and we chose to use an aluminum firewall. I think that it is 0.030"� thick. Light, simple, easy, and effective.
I don't think that teams should be able to use a composite firewall unless they have very good answers to questions about Tg (glass transition), Tm (melt), Td (degradation), max service temp, flammability, chemical resistance, and solvent resistance of both the matrix and reinforcement.

That is probably more lecture than any of you were interested in.

[Sam]

to elaborate on frankie.

we used the seat as a firewall. but the seat still got damn hot!! we put a thin sheet of ally in between the seat and the engine as a "Heat sheild" as it was only a small piece and did not seal out the engine bay, just to reflect some heat.

[Frank]

James,

do you know if you can "wet lay up" with phenolics?

i assume then it is a nasty process

do you think it wise?

thanks

Frank

[James Waltman]

Frank,
I am not familiar with anything like that but it could be possible. I will see if I can find out how it is done if it is not compression molded. I suppose that you would want to use a Resole phenolic resin. Resoles are one-part liquid resins that cure by heating (kind of like a pre-preg epoxy). Resole is often used for coatings – like on honeycomb. I don't know if you can buy it for wet lay-ups or not. It is a different cure process so it might not give off the nasty chemicals but I'm not sure.

If you were really ambitious you could make your own phenolic. Dr. Leo Baekeland made it for the first time nearly 100 years ago. You just add phenol and formaldehyde and you have phenolic. But why stop there. You are already on your way to Carbon/Carbon composites. You start with your carbon fiber reinforced phenolic. Pyrolize it (high temp no oxygen) and convert the phenolic to carbon. Then use chemical vapor deposition to densify it because pyrolysis leave voids. You can use methane to leave behind carbon. Then you have a composite part that can withstand several thousand degrees.

Really though, I think that aluminum is the way to go but I like to try new stuff.

[Didier Beaudoin]

I heard that the phenolics are very hard to work with because they retract a lot while hardening. Therefore, it's very difficult to make precise parts with it.

I heard the reaction produces water, which isn't exactly good for those who use a steel mold, which shouldn't be the case in Formula SAE anyway.

[James Waltman]

Didier,
I am going to assume that you mean shrink when you said retract. Nearly all polymers shrink when they cure (or cool). It is often harder to get parts off of male molds because of this. This is not a big deal if you keep this in mind when designing the mold (it can actually be an advantage). Phenolics shouldn't be much worse than most other plastics. How precise do you want your part to be?

The cure process for novolac phenolic produces dihydrogen monoxide as I listed above. The chemical formula for dhmo is probably familiar to most everyone: H2O (aka water). The molds get release coated and that helps to protect them and allow for easy part removal. The water is usually in a gaseous form (steam) and the molds are usually hot so there is not much chance of having them sit in water. I don't think that rust is a big problem.

[MikeWaggoner at UW]

Correction:
"The Tg is the temperature where a material starts to flow. It is a common term for polymers. For example polyethylene, ABS, PVC, and polypropylene also have a Tg. The matrix material is commonly a polymer (epoxy or polyester) and these generally have a low Tg."

Tg is NOT the melt temperature James. PE, for instance, is utilized above it's Tg (which is well below room temp). Tg is the temperature at which the plastic switches between brittle ("glassy") and rubbery behavior.

(ps - message edited to make Denny look crazy)

[This message was edited by MikeWaggoner at UW on March 11, 2004 at 07:33 PM.]

[Denny Trimble]

Do you mean PE is utilized above its Tg?

[James Waltman]

Yeah sorry Mike, I know. Wrong choice of words. Thanks for clarifying it for me.

[Denny Trimble]

I think all the dihydrogen monoxide in this corner of the states is getting to us...

[dancin stu]

Having used wet lay phenolic resins before with glassfibre, I would not recommend it to anyone! It is horrible stuff to work with. Unlike polyester, it does not break down the binder between the fibres, and so is reluctnat to conform to any shape, also the fumes are vile.

Produces nice, bright orange components though!http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Jarrod]

I know its not specifically on the same topic, but I was surprised at the Oz comp this year at the number of teams that had no firewall between the fuel tank and exhaust. I know the rules don't say anything about it, but for peace of mind surely you would run one? Any one have comments?

[Michael Jones]

Learn something new everyday...this time it's useful for a change http://fsae.com/groupee_common/emoticons/icon_wink.gif.

On that note, I've always found Tg applied to the weave and resins in tandem confusing - so, if you have a low Tg resin on fiberglass, is the resin the weakest link in a heat situation? If so, using standard epoxies without heat involved in postcure still will cause havoc, no?

Depends on what is done with respect to the seat and firewall combination, but I'm envisioning from the initial post something that will need to take considerable force (e.g., a driver strapped in and pressed back into said seat/firewall at 1g or so.) Something reaching rubbery consistency back there sounds like it will deform with this load, rendering the seat/firewall useless regardless of it being Kevlar or 'glass, if indeed the resin is the weakest link.

Lecture away, I'm a bit rusty learning about this but it's always interesting stuff.

[Garbo]

We did a quick burn test comparing Wet lay epoxy-CF with a 1/2" aluminum honeycomb core to 0.032" aluminum sheet. We used a Propane Torch for heat, 2" away from the surface.

Disclaimer: This test was not very scientific and was as much a late night "Let's play with fire." as anything.

Structurally, the aluminum was affected very little after 5 minutes. The aluminum did heat up to several hundred degrees (6 inches from center of flame, on 'driver side' of the sheet) in seconds.

The epoxy burned out of the hot side of the sandwich, starting 10 seconds or so after the flame was applied. The result was bare, brittle carbon cloth on the hot side. The temperature on the cool side never changed more than a couple degrees in 5 minutes and the cool side laminate which appeared to have unchanged properties.

The thought at the time was "I'd rather be sitting in the carbon seat." on the other hand, the rapidly heating aluminum seat might prompt you to eject, even if the track workers were asleep.

Garbo

[James Waltman]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Garbo:
The epoxy burned out of the hot side of the sandwich, starting 10 seconds or so after the flame was applied. <HR></BLOCKQUOTE>

Garbo,
Seems like a good reason not to use it as a firewall. You have all of the correct elements there just not in the right combination. Use aluminum skins on your aluminum honeycomb and you'll have the best of both worlds.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Michael Jones:
Lecture away, I'm a bit rusty learning about this stuff but it's always interesting stuff. <HR></BLOCKQUOTE>

Michael, you asked for it.

The resin is almost always the weakest link for a fiberglass composite part when heat is involved. S glass has a max melting point of 1725C. I think that it gets soft well before that but you get the idea. A resin that is good to 100C on a fiber that is good to 1000C. As an example of the high temp capability of fiberglass: we are using some E glass weave as temporary header wrap.

Standard epoxies without a post cure would be hard pressed to stand up to engine temps. As one of our professors is fond of saying "it is all about the crosslinking"�. Epoxies are thermoset polymers that cure by crosslinking (condensation reaction). During the cure process the viscosity of the resin increases once the molecule(s) start to crosslink. When the viscosity increases it makes flow more difficult and it makes it more difficult to continue crosslinking. When you post cure (or cure) a part at an elevated temp you give the resin some extra energy (heat). This allows some movement and allows more crosslinks to form. Once the crosslinks have formed they stay. The higher the crosslink density the higher the Tg and generally better mechanical properties. The resin's ultimate Tg is when the maximum number of crosslinks have formed.

Polyester resins are a little different. They cure by an addition reaction. This is like a chain reaction. That is why it only takes a little bit of initiator to cure large quantities of polyester. Polyester resin contains a monomer. The purpose of the monomer is to form crosslinks. Typically the monomer is styrene. Styrene gives polyester the smell so many of us have grown to love. Chlorostyrene is used as a monomer in some resins to help improve the flame retardant properties. At room temperature the polyester gets about as crosslinked as it ever will.

Thermal cycling can be a real problem in composites. Raise the temp, lower the temp and repeat. Sounds a lot like fatigue. The temperature swings can set up large cycles of internal stress. This can lead to problems even without load on the part. It is probably not a real concern for any of our applications but I'm on a role...

TGDDM is a good epoxy for high temp applications but is a nightmare to work with (think hot melt glue). It has been used here for one of the projects that required a resin system that could stand up to high temps. It is the part in the middle of the following picture. This picture is a little old and a bit more progress has been made. I put this picture up on our website a long time ago and hid it in with pictures of the V8 (that's the V8 in the back). I figured it would generate some interest but nobody asked me about it until Tim Heinemann did a few weeks ago.

http://dot.etec.wwu.edu/fsae/v30/image045.jpg

This summer I got hired on as a research assistant to help build the body for Viking 32. Viking 32 is a full size car that we are building for the Dept of Transportation. The body is all carbon fiber. We post cured the whole thing once we were done. I will see if I can post some pictures later of our set up for post curing Viking 32's body.

Hope I didn't slip up and make any mistakes in this rambling. Feel free to call me on it if you find something.

[Michael Jones]

Once again, I learn something new every day, and once again, it's interesting. Thanks much! It at least makes logical sense based on what I have discovered in my scanning the literature and filled in a few gaping holes in my knowledge of composite mechanics.

We're not particularly strong in that here generally speaking, and I'm as much of an engineer as the doctors on ER are doctors, formally speaking, so I'm bound to miss a few details.

On thermal cycling, what temperature would this start to matter? I would assume at the lowest Tg of the combination of resin/curing technique and fibers used.

Given that you don't see this as much of concern, I suppose the number of cycles to failure is too high for us to care much, given the special-purpose nature of these cars. That noted, you do suggest the E-glass header wrap is temporary, which suggests that number is not, say, the fatigue life cycle of steel.

I would also guess that cycles until failure will decrease if it did bear a load of some magnitude.

So, in our firewall/seat example posed, would this come into play? There's heat (and localized heat - I'd imagine that's an issue as well, since parts of the glass wall would be much cooler than others, near headers for example) and by definition the application of the heat is cyclical, but there's not all that many cycles over the course of use in a year. And there's a force pushing back...

Hm. I'm beginning to be glad that we didn't do this three years ago. Even if it worked, we would've had no idea why. http://fsae.com/groupee_common/emoticons/icon_razz.gif

[RagingGrandpa]

Here's a question- Has anyone had PROBLEMS with a seat/firewall combo?

[James Waltman]

I'm not sure what temperatures or number of cycles the fatigue starts to be a problem. Your guess is as good as mine. I say it is probably not a concern for us because the cycles should be relatively low assuming that you are operating within the capabilities of the composite system. I think that aerospace has trouble with this for things in orbit - massive temperature variations.
The header wrap is only an example of the temperatures that fiberglass can take – no resin just glass. It's not structural and it doesn't really matter if it falls off. It's temporary because we are planning on using real header wrap when we can afford to buy some more. We used E-glass is because it is inexpensive and we have a ton lying around. I'm pretty sure that we will burn through the headers long before the glass has any problems.

Of course I could be very wrong about all of this. I know that some of you guys have more knowledge about composites than I do. Maybe someone else will share some insight.

(ps – message not edited to make Mike Waggoner look crazy and he still does)

[Garbo]

James,

Good call on not using the resin as part of the firewall... since I could crumble the hot side of the composite by hand afterward, there is the distrubing possibility that engine compartment heat could result in some nasty road rash...

Maybe to clarify, we're not using either of the tested firewalls, something much more fun is on the car.

Crosslinking; I have only delved in to this on my own time so be warned. Isn't curing at a set temperature (defined by the resin) required for all epoxies to complete cross-linking? And doesn't the epoxy start to break down slightly above the cure temp? For instance, we are using a bunch of structural prepreg which cures at 175 celsius and is safe for use up to 230 celsius. What do you mean by increasing the temperature resistance by post-curing?

Also, cross linking; my understanding is that cross linking is initiated at a temperature below the cure temperatuer and you hold this temperature for a set amount of time to allow cross linking to finish. Raising the temperature to the cure temp. stops cross linking, doesn't it? Or is this what you mean by 'post-curing' and I'm confused because we've been doing it as a single process?

[Michael Jones]

Temperature cycling also comes into play in less volatile heat conditions but the number of cycles is huge. So, things like 'glass/asphalt roof shingles last longer in temperate conditions than warmer climates and consistent vs. radically varying climates, all other factors being equal, so I'm told by a friend of mine in construction. But that's a question of 15 vs. 20 years, so nothing for FSAE teams to worry about I guess. If these cars last beyond a few years, it's good news.

I'd suspect that a firewall/seat real life test might work, but Murphy's Law would suggest that it would fail only in the endurance event. As as James suggests, it would certainly help in design having a strong theoretical justification for such an effort, or you will look like an idiot who just slams things on for the hell of it.

Nothing wrong with doing that and coming up with the justification later, of course, but the justification is important.

I guess the temporary E-glass header wrap is just held on in tension with itself then, like a bandage? Interesting, and given the temp of headers, certainly shows that E-glass is pretty resilient at high temps. I'd imagine equally that if it was affixed by resin, it wouldn't be for long. Would be an interesting test in its own right. And probably quite toxic. http://fsae.com/groupee_common/emoticons/icon_razz.gif

[James Waltman]

A cured resin does not necessarily have the maximum crosslink density. This comes into play mostly for resins that will cure at room temp but may have improved thermal properties if they get an elevated temp cure (or post cure). Garbo, you mention break down. This occurs at the degradation temperature. It is possible to reach the ultimate Tg without reaching the degradation temperature, although I suspect that these temperatures are often pretty close.
Pre-preg behaves in a similar way. They will "cure"� at room temperature if they get enough time. That's why pre-pregs (and other one part epoxies) are stored in freezers and in a production environment the time they spend out of the freezer is carefully recorded. Eventually they expire because they have cured a little. So pre-pregs are always crosslinking. When you add heat they cure much faster. Pre-preg already gets cured at an elevated temperature so post cures are not necessary.

Garbo, I think that you are talking about a ramp schedule for pre-pregs. The temperature changes are there for other reasons. Once you add heat (or the hardener in a two part system) the crosslinking is going to take place and continue until it is complete.

These are some pictures of our elevated temp post cure for Viking 32. We did all the lay-ups with a two part room temp cure epoxy. We did the post cure at about 150 degees F. The mold was fiberglass/polyester. The body is all carbon fiber with a Rohacell core. We tried to make a big oven to do the post cure but ran into some problems. So we turned the mold into an oven.

The vacuum bag
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/Vacuum%20bag.JPG
Jimi and I made the vacuum bag and I'm proud to say it didn't leak. I think that we made 2 bags this big and 4 more that were about half the size. They were the biggest vacuum bags I had ever seen until we took a tour of Boeing a few months ago. They do some massive parts.

The Mold
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/The%20mold%20before%20post%20cure.jpg

The insulation
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/With%20insulation%20for%20post%20cure.jpg

The final product in primer
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/DSC04965.JPG
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/DSC04970.JPG

[RagingGrandpa]

might I ask how the hell you made the mold for that beautiful shell there?

[James Waltman]

It started as a 10th scale model. That went into the CNC mill got digitized. Those points went into a CNC router and stations were made. The stations were full size silhouettes of plywood. I think they were spaced out every 12 inches. They space between the stations was filled in with high density foam. Massive amounts of Bondo smoothed everything out. A case of the body filler had 4 gallons and there were a lot of cases. Then it got primer and paint. The result is a full size model.

Fences were added and the mold was made of fiberglass. The reinforcements in the picture above were steel tube. The majority of the mold was three sections – 2 sides and the top. Two smaller pieces made the front and rear. You can see the sections in the picture above.

The plug:
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/Image18.jpg
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/plug%203.jpg

The plug with fences:
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/plug%20fences%201.jpg

Fiberglass on the side makes the mold:
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/tooling%20layup%201.jpg

I recommend that you laminate a picture into any mold you make. This side featured a poster of Bobby Brown and the other side had a poster of Brett Michaels of Poison. A polyester matrix Bobby Brown reinforced composite has remarkable mechanical properties.
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/Bobby%20Brown%201.jpg

The body just came back from paint yesterday. It should be mounted on the chassis in the next few weeks.
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/DSC05437.JPG

I should point out that the mold was 100% student done. I didn't help make the mold but I was around while it was being made. I did help make the body. By the end of the summer I had sworn off composites but now that has worn off. Dr. Seal had a pretty good saying when we told him that lay-ups were getting old.
"There are two types of people in the world: those that don't know how to do composites and those that know better."�

[This message was edited by James Waltman on March 25, 2004 at 11:39 AM.]

[James Waltman]

While I'm on a role I figured I would post some other pics of Viking 32. Sorry to hijack the thread.

The plug in primer
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/Wheel%2520Well%25203.jpg

Toying around with the wings for the rear. We were inspired by the superbird.
http://dot.etec.wwu.edu/fsae/SharedPictures/V32/Superbird.jpg

http://www.musclecarclub.com/musclecars/plymouth-superbird/images/plymouth-superbird-1970i.jpg

[ozzy]

Just finished reading the thread, a very good read!! You guys really know what you are talking about!

Amazing mold and lay-ups. A very good job in the whole process. I have done a few molds before, but nothing as big as that!!! I am doing the body on our car this year, so that will be very good, i am looking foward to it.

keep the lessons coming!

[Travis Garrison]

I wouldn't jump to any conclusions Ozzy...James did try to make a super bird wing with some masking tape, taking lessons from him may lead to bad things...

Travis Garrison
WWU FSAE

[Garbo]

Hey, tape is the best wing material. There is a Cavalier at our school with a wing built out of yellow duct tape. I think it adds like 50 horsepower. He also has four yellow 'speed dots' taped into his rear window... for extra downforce...

I'll try to get a few pics.

g

[RagingGrandpa]

wow that is an amazing mold, I know how much damn time we spend bondo-ing our routered pink foam nosecone mold, and that car is umpteen times larger. Happy cancer, but it looks great!

[Michael Jones]

That is absolutely fantastic. I'm amazed and humbled, especially since I can't seal a vacuum bag to save my ass half the time. We really need a new pump anyway. We're simply not set up for quality work here.

If I ever decide to become a real engineer as opposed to faking it somewhat convincingly, I'll be sure to consider WWU just to get a chance to make such things. http://fsae.com/groupee_common/emoticons/icon_smile.gif